I will never understand why people hated this episode. I personally found it brilliant to teach an important lesson: Know your friends well enough to know WHAT WORKS AND WHAT DOESN'T.
Rainbowdash has been shown countless times to act irrationally and doesn't do well with lectures or heeding warnings. Sure, at times she will listen when there is a serious need for it and she'll know that her way isn't the best, but in this case, fame was getting in her head and feeding her ego.
Whenever an individual is caught up in the moment, they WILL GET OUT OF CHARACTER. At least with most people especially with a personality similar to Rarity's and RD's. This isn't a BAD thing though....it's merely a common observation people need to understand.
Were they reckless for bashing down RD's ego? Yes. Was it a bit over the top? Yes. Did it work? YES. Why people saw it as cruel, I'll never understand. Obviously, any RD fan SHOULD know that RD wouldn't take a talk seriously if they were telling her she was becoming full of herself and need to take her ego down a few notches and not take her act of heroism as some fame-seeking stunt.
This proves they knew her and knew how to Pierce that skin of hers. Telling someone they are wrong is hardly ever easy and it's even harder to ACCEPT your faults as well. Their method worked....it's called tough love. It wasn't even cruel....they simply one-uped her which is something RD has no problem doing to others apparently whenever there is some sort of competition or challenge.
You know what would have been cruel and even more reckless? If they sabotaged an attempt from her causing ponies to get hurt then blame RD for failing. That would have taught her a lesson but it would have been a dick move and unforgivable.
True....sometimes indirect confrontation isn't always the best way to go in certain situations. It seems like a trend for the teacher to always leave the student in the dark till the very last second which can either end good or bad. Which van be frustrating for both the fans and student.
But in this case, it wasn't as terrible as some make it out to be.
Mare Do-Well was a good episode in my opinion.... great way to highlight how much they know a friend and what needed to be done even if it wasn't "easiest" or "rational" way to go.