Since the start of Season 2, Rainbow Dash traditionally has the worst episodes of the season and show at large. After a swarm of some great and excellent outings, Season 9 releases its first clunker.

1. Smolder had one of her best outings of the season. Representing the student body who wanted the cheerleading to work, she put up with all of Dash's crap and gave her the calling-out she so rightfully deserved.
    
2. After Dash gives an apology that didn't sound so contrite, she actually proved she meant it by helping her students practice. And if you look at the montage, you'll see how much effort Dash put into coaching them to be the best they can be and the students placing their trust back into her.
    
3. Snips continues his capitalist streak, trying to sell as much as possible to buckball fans and goers. And he inadvertently made Dash see the errors of her ways.
    
4. Celestia going all fanatical remains the episode's funniest moment.
    
5. And the buckball scene as a whole (minus one moment, which I'll get to) holds up well enough.

That's all the positives. The rest of this episode was just a colossal train wreck.

There's one piece of dialogue, where after it all ends, that really spoils the mood.

Quote

Twilight Sparkle: Wow, Rainbow Dash! It's almost like you were the perfect pony to coach cheer squad after all!

Rainbow Dash: Wait a second. Was this one of those things where you knew the lesson I needed to learn before I learnt it?

Twilight Sparkle: You mean like how you don't have to care about a thing other ponies like as long as you care about the ponies who like it? Nope. Don't know what you're talking about.

And attached with a snarky, conceited face and tone.

So why is Twilight going all Trollight the worst moment of the season so far?

1. It calls back memories of what the RM5 did in Mare Do Well and 28PL, the former one of the most infamous of the entire series. Rather than tell Dash upfront, they go behind their backs just to make her get it, and the tone attached to them is cruel and unbecoming of not only anyone who truly cares for a friend, but also the show. For those two, it took until being confronted by them at the very end with them actually telling her to her face. 28 Pranks Later has the worst atmosphere of the two, because rather than fix the problem, they exacerbated MDW's worst problems and made a worse version of that episode. Here, Dash learns her lesson well before the climax, and Twilight doesn't see the transformation taking place, but imagine if she didn’t. What if Twi told her at the end? Can you imagine just how dirtier that would feel to the audience?
    
2. Just to teach Dash a contrived lesson, Twilight intentionally put the whole tournament at risk. Had Twilight decided not to make Dash the cheerleader coach beforehand, none of what happened would've existed. Dash would be at home teaching buckball and not feel demoted and deflated. She started a conflict that should never have existed in the first place.
    
3. But what does making that snide, passive-aggressive line admission also mean? Twilight knew well in advance that Dash would not take this “demotion” very well, do less than minimum effort to help the students prepare for the halftime show, try to sneak away and get involved with it, potentially ruin all hope they have for it, and make them quit cheerleading practice. Like the Ponyvillagers in MDW, she used both Dash and her students as guinea pigs just to make Dash learn a lesson. This is one of her most out-of-character moments of the whole series, as it shows no trust with Dash and the student body. That final exchange is an admission of no confidence, yet the episode paints her in the right for pulling such a disgraceful stunt.

Trollestia was the worst part of both Ticket Master and Bird in a Hoof, as she treated the Mane 6 (and for the latter, her sick phoenix) as a means to an end. Trollight adopting that same "quality" doesn't make it any better or more humorous.

Because this episode's Mare Do Well 2.5, it's easy to see why many don't take it very well. Mare Do Well is factually terrible, and its infamy means it should've been put in the background and not to be reminded of again. This episode and the ending feel a lot like a multi-down and an admission by DHX of having continuous difficulties writing/editing her well.

Those reactions also remind me of my own following Princess Spike's ending. After a mess of an episode and conflict, Spike was given a bouquet of Dragon Sneeze trees, restarting his allergies and forcing him to sneeze at the rebuilt statue. What made that moment so atrocious is how it solidifies a very sexist position within the show. Spike had been the show’s buttmonkey from the beginning, and several episodes either neglected him despite being an important part of Twi’s life (like not being at Twi’s birthday) or made him the butt of very unfunny slapstick (Owl’s Well, the Spikeabuse from Fall Weather Friends and Castle Mane-ia). Additionally to being the only non-pony of the Mane cast, he was the only male. Beating down the only male lead in a pro-feminist show is as misandrist and anti-feminist as it comes.

Now, is Twilight’s line as bad as that atrocious, sorry excuse for a "joke"? Not even close. PS's "joke" made me wonder whether FIM jumped the shark, a dose of irony after Slice of Life subtly satirized it. Twilight's admission, as awful as it is, isn't nearly that low. To this day, I still don't regret my tirade over it. Thankfully, Spike had more than half a season, several more from that point forward to recover. From that day forward, he hasn’t had an episode close to this level of quality since. Unfortunately for Dash, we’re now in the last season, and if you look at the synopses of what’s to come,

Also, Kaita Mpambara is one of the best new writers, so for him to write such a stinker is massively disappointing.

However, just because Twilight intentionally placed Dash in an unfavorable position doesn't mean Dash is entitled to take her disappointment out on the students. With RD being the focus, we see how she feels and reacts to her surrounding. What we got here is Rainbow Dash's Honest Apple.

1. In Honest Apple, Applejack wasn't initially sure she would be the right pony to judge on practicality, but after Apple Bloom (inexplicably) had trouble with her hat. So even though Applejack became Applejackass, it didn't start out that way. She went into this process without ill intentions. However, Rainbow Dash never got on the right foot with Ocellus, Smolder, Yona, or the two cheerleading valley mares (Shimmy Shake & Lighthoof).
   
   What was her reaction to Twi over its importance?

   Quote

   Dash: *guffawing*

   …Nice support for your students, Ms. Cools-a-little.

2. From the get-go, Rainbow Dash not only showed absolutely zero interest in teaching the students how to cheerlead, but made less-than-minimal effort. Let's go over her sins one by one, shall we?

   1. Shimmy Shake and Lighthoof were apparently students at the school. But Dash apparently doesn't recognize them very well and passively dissed their cheerleading routine by rolling her eyes at them. However, even if Shimmy and LH may not interest her, she could've been inspired by Ocellus's desire for respect, Smolder's subtle feminine curiosity, and Yona's eagerness to fuel her in helping all five practice. Instead, what does she do? Remain completely apathetic towards cheerleading as a whole, ignore Snips's (bit-centric) reminder that Twilight put her faith in her, and put her focus completely on the constructing buckball field over her own classroom.
      
      Their first practice in front of her was loaded with problems, including Yona's inability to not cause a classroom earthquake, Ocellus's timidity, and Smolder's poor smoke direction. But Dash pays no attention, her focus out at the window. She doesn't see one second of it. As far as she's concerned, as long as they just perform in front of her, it's all she and the buckball audience would give a shit about.
       
   2. As long as someone else with more passion than her can teach them, then she can watch ponies practice and build the field. Even though Snips's focus is bit-centric, he showed to be no fool these days. However, his vague wording of needing a coach so he can "make any bits" led her to her scheme, which was a disaster.
      
      
      
      …
      
      …
      
      Seriously, Dash? You really think everything will be A-OK? You think that they will rather have Snips, who's completely unqualified to teach or tutor in any form of athletics, over you?
       

   3. Quote

      Rainbow Dash: So don't dance and cheer.

      Yona: Uh, then what cheer squad do?

      Rainbow Dash: I dunno.

      Smolder: [angrily] Maybe you should! [calmly] Or whatever. You're the coach! You're supposed to come up with stuff for us to do.

      
      
      Yet, she uses that "come-up-with-something" idea to blindfold her students and try to sneak out, and would've succeeded had Yona not peaked. "Come up with something" isn't an excuse to be damn LAZY and do NOTHING under a cheap guise.

   4. After being suggested to "turn to her friends for help," she did just that and spent the rest of the first day of practice asking her friends. But she gets only the equipment. She literally is doing "what she needs" just to get stuff. No tips on how to work with them safely, arrange them properly so the practice and the dance routine improve, etc. In layman's terms:

      Spoiler

      Dash: *unveils equipment* Here y'go! Now spend worthless time preparing for a cheerleading practice without me. I'll be where the real fun is! Toodles! *flies away*

      Lo and behold, they practice for the rest of the day and fuck everything up in the worst "comedy" routine of the season! What does Dash do? Completely ignore it and act like it never existed. When she turned around to see them all glum and upset, her only reaction was a disinterested "What?"

   5. Hmmmmmmmmmm…

      Quote

      Rainbow Dash: I don't know what everypony's so upset about. So it wasn't great. Cheerleading's never great. That's why everypony gets up to get a snack during it.

      …now what does that remind me of?
      
      Oh, yeah, this little shit!!

      Quote

      Applejack: No! There is no choice! They're the exact same belt! *slams hoof to ground* Ugh! This is so silly! Fashion is ridiculous!

      Applejack: Why are you mad at me?! I was just sayin' what I thought! And isn't that why you wanted me to be a judge?! For my honesty?!

      AJ's act of shaking all those feathers off Lily Lace's hat after uniquely stitching them one by one overnight is by far the cruelest act between the two episodes, but Dash's words were much worse. In HA, AJ stereotyped fashion at a boiling point (though her following words while much more composed doesn't help her at all) and at least gave it a chance. Here, Dash maintained a negative, narrow, stereotypical viewpoint of cheerleading before Twilight assigned her, maintained it throughout, and then remorselessly showed her disgust for it and those willing to perform and make it good in spite of her laziness! Even worse, despite being visibly upset, she still didn't see the big deal in how hurtful her actions were!
      
      *facehoof* Sweet Celestia!

3. Look, I get it. Being assigned to a job you don't like sucks. No one likes it. But this isn't about you. It's them. THEY agreed to take part in the School of Friendship's cheer squad, because THEY were interested and knew Dash was the most athletic and best one to properly rally! Dash, you're a TEACHER. It's your job as a TEACHER to HELP THEM! It's one thing to be so oblivious towards their screwups. It's another to severely flanderize her ignorance, pretend they don't exist, act so disinterested when they badly mess up, directly insult them, and STILL don't care!
   
   I read one comment somewhere, and I can't find it, but that brony's right. Dash, why are you a teacher? This is the second time you were a selfish piece of shit. What you did with AJ on that boat is way, way worse than this, but that doesn't make your despicable, out-of-character actions here any better. If you truly needed help like you claimed, you coulda, I don't know, look in the library yourself and work with the students to hone their craft! Neighsay bashed the school in part because the teachers are professionally unqualified. Congratu-pony-lations for proving him right again!
   
   *AAAAAAAAAAAARGH!!*
    
4. Ain't it odd that Dash suddenly bashed the idea of cheering? IIRC, she had no problem teaching someone how to cheer eight seasons ago!
    
   
   Sure, teaching someone how to cheer from in the stands is much more different than cheerleading, which is pre-planned and organized. That doesn't mean she can't use those lessons here, but that would've been a leeeeeeeetle too helpful for her students, wouldn't it?

Needless to say, this episode sucks. Today, it's the worst of the season and yet another Dash-centric flop.

However, despite bashing Dash's characterization and discontinuity, it's not close to the worst of the show or her worst appearance ever. Several episodes prior handled her way worse than this.

1. May the Best Pet Win, Tanks for the Memories: Abusive to animals. The former by being cruel to those who want to be her pet and dismissive toward Tank. The latter for treating her company with Tank to be more important than keeping her alive. Sorry not sorry, Dash DOESN'T deserve pity over having to wait three months during the winter!
    
2. Rainbow Falls: Gaining an ego and then actually thinking about abandoning the relay team representing her town in favor of the “cooler” Wonderbolts squad.
    
3. Mare Do Well: No explanation required.
    
4. 28 Pranks Later: Jumpstarted the MDW ripoff by scaring the daylights out of FS while knowing she hates being pranked, and then the whole town, including SCOOTALOO, decided to get her back.
    
5. Newbie Dash: What supposed to be her milestone episode turned out to be the biggest torture porn of the entire show. Three years later, it remains FIM’s biggest black mark.
    
6. Compete Crap Clause: Lusting to win Teacher of the Month, she and AJ focus more on winning than the Young Six’s safety, causing a Yona to nearly drown! Rather than learn their lesson, they act passive-aggressive toward each other and nearly became bite-a-cuda dinner! 

When Rainbow Dash wised up, she was remorseful, apologized, and worked hard to help the students make the cheerleading become the go-to moment of the tournament. Despite her terrible behavior, she put in the effort to make up for it, a commendable act by her. Yeah, it’s the worst episode this season, but compared to almost the rest I listed, it’s not terrible.

Lastly, it's understandable that some may feel DHX, particularly the crew from S6 onward, hates Rainbow Dash. But if they do, then we wouldn't have the following:

1. Stranger Than Fan Fiction: OK, this episode isn't as good as what many say, but only due to Quibble suddenly acting like an idiot while in the jungle (thinking it's all a game). OTOH, Dash was fantastic as a fantastic foil.
    
2. Top Bolt: A better climax could've really helped this episode. But Twi and Dash worked hard to help out Sky Stinger and Vapor Trail and were able to fix their friendships. Their portrayals were among the best of S6.
    
3. Glideance: Dash’s Putting Your Hoof Down, and done right.
    
4. Grannies Gone Wild: An episode that has no business being great, but it is. Initially wanting no part in it and trying to skip out, she immediately sees the error of her ways and works tirelessly to keep the Golden Mares safe. Despite all of the comedy at its expense, Berrow's episode reminds us to sympathize with her, not think she had it coming. She rightfully earned her way to the roller coaster.
    
5. The End In Friend: NCC and Mare Do Well done right. Their argument was very believable and had every right to defend their own interests when the other didn't take theirs seriously. But they used their interests and strengths to realize how much they still mean to each other.
    
6. The Washouts: An excellent sequel to Wonderbolts Academy, and Dash’s best episode. Like its predecessor, it balances her strengths and flaws very well, transitions between Dash's insecurity and worries for Scoot's safety flawlessly, and doesn't demonize her for any of it.
    
7. Common Ground: An amazing followup of STFF with a personal touch, and Dash's best outing of the season. Tries her best to help Quibble learn buckball to bond with his stepdaughter, accidentally messes up, and steps back up to continue helping him until she found the right resolution.

Most importantly, as disappointing as 246G is, this is Season 9's only folly. Many more episodes remain. If you wish, you can open the spoiler box beneath for brief commentary on the early-aired episodes:

So, yeah, it's a bad episode, but not the worst, and whenever the show ends, she still has many good episodes.