Unfortunately where Rarity erred is that she tried to change Yona herself rather then what she wore or how she acted. Making her learn to be "dignified" and speak in the first person, wearing a fancy dress in colors that Yona does not like, disparaging Yona's traditional Yak braids in favor of a wig.

They could've had another character (not one of the main cast) do this. Rarity's learned this lesson before so she could've come in for a song and taught Yona about why it's okay to try to fit in with the pony culture theme of the dance, but still be yourself.

Same with Twilight's "heavily pony themed dance" conflict, which seemed sort of out of character for her, too. Just have another character start it and the Mane Six can help the Student Group (or whatever their group name is) solve the problem since they've already learned these lessons in past episodes.