Oh yeah I forgot that Diamond Tiara was reformed...not a very big fan of that, like at all. Kinda feels like they were trying to pull off a Pacifica redemption from Gravity Falls, but not as well planned out or anything like that. I never really cared about Diamond Tiara, like at all. I agree that not everyone is going to be nice to you and Diamond Tiara (and Silver spoon?) should've stayed bratty little bullies.
Also I'm not sure how many villains are reformed ^^" I kind of stopped watching the show like a long time ago and now I only know what's going on due to random clips I see. Here's my count of which ones got reformed:
Luna
Discord
Starlight (BUT WHY THOUGH!?!?!?!)
Sunset
Diamond Tiara (and possibly Silver Spoon???)
I think that's about it for like, major baddies. I'm not sure about the little dudes who didn't really matter all too much, although I am aware I am missing some.
I would count all of the Changlings, but I don't feel like they were totally bad because they were both following their queen, and doing what they had to do to survive.