Thrond

@Them's Seeing Ponies 

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Pinkie doesn't function the way the rest of the six do, and I think the writing staff struggles with this nowadays; they want to prove to the peripheral fandom that they're pumping out an emotional and 'mature' series, but they can't use Pinkie in the same way, nor can they replicate the brisk, comedic Craig McCracken-esque style that fit Pinkie so well in the earlier seasons. 

It's the fault of Pinkie character design in the first place that hinder her future character development, or... the incompetent of the writers? Every original cast in early seasons is some cartoon archtype characters you can find in TV Trope, Fluttershy is the shy one, Spike is the butt-monkey, Rarity is fashionista etc then eventually, they changed to something more than what stereotype define them but still keep their charms. What did they do to Pinkie? They made her the best character in Pinkie Pride then make her stupid in Filly Vanilli (the episode right next to Pinkie Pride), and they were written by the same writer, that shows how they handle Pinkie. Maud used to be a joke character just to add dry humor to the show, and look what she is now, a rock specialist, befriend with Starlight, she loves her sister but can get tired of her shit sometime, got a boyfriend and smile more than often, that's the progression, what are their excuses? She isn't even the main character!

In the movie, Pinkie can work in comedic and dramatic moments just as fine as everyone else, I dare to say she is one of the best part of the movie (and I even dare to say she should be main character of the movie instead of Twilight Sparkle). If they cant make her funnier, then make her deeper. If you want a character grow, put them into a tree and throw a bunch of rock at them, keep doing that and it will works. Throw a reality checks to a happy-go-lucky character can strengthen their character, but I think the writer afraid of throwing any rock, remember Yakity-Sax? Status Quo is so unbreakable in season 9.

In short, I blame the writers.

@Lambdadelta I was trying to point out that it was more the incompetence of the writing staff than Pinkie's original design. I think there's more potential for Pinkie than the staff seem to realize; it's just that I think they're using the wrong approach instead of expanding her character while staying true to her Faust-era traits (which was a fun-loving optimist as seen in episodes as late as Pinkie Pride and Lost Treasure of Griffonstone as opposed to a flat hyperactive figure, which episodes like Yakity-Sax seem to interpret her as). While it depends on opinions of how the later seasons have handled the cast, I feel the writers have basically sunk into a rut with Pinkie, as if they're unsure on how to move her forward without a concrete 'goal' to achieve (the same applies with AJ - notice how they spontaneously introduced a goal for Fluttershy after seven seasons largely to create this sense of progression). Not to say she hasn't budged an inch since the pilot - Party of One and Pinkie Pride are both excellent examples of Pinkie development as both challenge her philosophy (the kind of conflict that Pinkie needs most to develop, as she lacks any long-term ambition to head towards) or, as you said 'throw stones' at her current flaws, whilst allowing her to maintain her essence of optimism and conveying a fun story simultaneously. However, the departure of writers such as Meghan and AKR seemed to have damaged Pinkie significantly as well, as the later writing team began to interpret her more as a generic 'wacky' character with huge emotional canyons (think her reactions to 'losing' Maud or taking criticism in Yakity-Sax), which stalled her progress even further than the McCarthy seasons (which I still like, but Pinkie's character in her non-focus episodes feels more awkward than the earlier seasons to me). 

I also understand your points about season 9. I suspect the writers want to give each cast member a kind of send-off, but the actual material they've released suggests that they're running dry of ideas for the six and resultantly resort more to ticking the boxes (Cheese Sandwich returns? Check!) than crafting a satisfying story or taking risks. It's a problem I've had with some of Haber's work in the past, but it seems more conspicuous here than usual thanks to naturally heightened expectations. 

@Them's Seeing Ponies

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By comparison, the show in S4 and beyond began to shift its priorities away from comedic slice-of-life capers into an attempt at a more full-blooded and moralistic fantasy series, which, while it works at least adequately for the rest of the six, simply didn't work all that well for Pinkie, as the show's focus now lay onto 'big', dramatic character moments and emotional scenes as opposed to comedy.

I never really felt that Pinkie stopped being funny, to be fair; I even liked the moments that annoyed other people. 

But yeah, she’s been subject to (in my opinion) the most baffling instances of the show contradicting itself, like “Yakity Sax.” I thought the show was still doing right by her as late as season 6, but these three latest seasons have just had nothing for her. This show makes writing for these characters seem like a Herculean task, even though they’re rich enough that the episodes should write themselves.

Personally I don’t think this approach works well for the rest of the mane six; it’s okay for Rarity and Rainbow Dash, because those two work best when they have something specific to worry about, but without the abundance of comedic digressions from the early seasons, they all feel simplified. The best moments in this show are still when we learn something new about the characters in a tossed-off gag, and that’s less frequent now. 

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they want to prove to the peripheral fandom that they're pumping out an emotional and 'mature' series, but they can't use Pinkie in the same way, nor can they replicate the brisk, comedic Craig McCracken-esque style that fit Pinkie so well in the earlier seasons. 

...sometimes I think this show needs to lower its pretensions sometimes and focus on the aspects of itself that work the best inherently.

I’ve felt this for several years now, and I truly believe that many silly episodes from earlier in the show are more mature than the serious stuff that came later. It honestly surprises me that this take isn’t more common, but I guess people who share my opinion don’t stubbornly cling to the show years after its peak.

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It's a problem I've had with some of Haber's work in the past, but it seems more conspicuous here than usual thanks to naturally heightened expectations. 

I find it ironic that Haber is such a weak story editor, because I think his writing is often very good. I think he “gets” the characters, but it sometimes seems like he doesn’t really care if anyone else does.

@Lambdadelta

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In the movie, Pinkie can work in comedic and dramatic moments just as fine as everyone else, I dare to say she is one of the best part of the movie (and I even dare to say she should be main character of the movie instead of Twilight Sparkle).

The movie might actually be my least favourite Pinkie Pie appearance ever. She was just noisy in that. 

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but I think the writer afraid of throwing any rock, remember Yakity-Sax? Status Quo is so unbreakable in season 9.

In short, I blame the writers.

I think the show has trended towards making the characters more boring since season 4; often, the only new angle they can come up with is some kind of job change. 

I wish this season had more of the friendship school. That was such a breath of fresh air.