Honestly, some of the redemptions are excellent and make sense, others do not. Starlight had a terrible reason for being evil (trust me, losing one friend whom she probably could have just written letters to is NOT a good excuse for stripping others of what makes them who they are and trying to undo the friendships of others), and she did some extremely HORRIBLE things. No she wasn't pure evil like Sombra or Tirek, but tbh, the finale would have worked much better if she had been because that backstory undermined her entire character, and the finale would have had more impact if Twilight had not been able to save her and instead had to banish her or something.
She could have been redeemed later after having served a sentence and actually having time to think about her mistakes and truly repent for them, but instead the message season 5 gives is "it's okay to hurt others because you're hurt as long as you're sorry for it later" and that's just BS. Not to mention that it's unrealistic for everypony to be redeemable since all people aren't. There are LOADS of people who don't WANT to be good, who don't care about others at all and who live for treating others like crap. People who are like that REFUSE to be reformed, and it's silly for there to be so few antagonists in the series showing this like it is.
I understand why they reform so many villains, they are trying to teach kids (and other viewers) that it is best to try to make amends with those who have wronged you, and it is best to try. However, some people will just continue wronging you if you continue to forgive them; some people, if left unpunished, will continue being criminals and will continue to deliberately cause others harm. Some people do not deserve forgiveness, and, for the weak reasoning she had compared to the crime she committed, Starlight Glimmer is a pony who really doesn't deserve forgiveness. Her redemption and backstory were handled exceptionally poorly, and that's quite sad because that finale episode was good until the backstory and redemption occurred. The story would have benefited more from Twilight having to accept that somepony so similar to her was somepony she couldn't be friends with, from having to deal with the fact that friendship, while powerful and able to conquer many things, can't solve every problem, and from having to do something as Princess that would be difficult to do (such as banishing Starlight) for the greater good.
That said, again I shall reiterate that I understand why they redeem most of the baddies and have fully accepted that they will continue to do this. The show isn't supposed to be realistic, it's fairly utopian. I mean most of the villains aren't even trying to actively harm anyone, they're mostly tame. But imo it would have been better to have a pony villain who was seemingly "normal" be irredeemable from a story/development perspective.