The show's title is literally called Friendship is Magic, and the lessons it preaches will no doubt always lean on the positive side—in this case, believing that anyone can be a good person regardless of the actions they've committed in the past. However, as most people have said here, the redemptions can potentially become unrealistic and overly rushed. The whole "villains reforming" concept could be really well done and still remain true to the central "everybody becoming friends" theme of the show, but unfortunately many cases of this were poorly executed.
Discord is, arguably, one of the most well done redemptions done in the series, since it clearly took time and effort for him to grasp the concept of friendship. And even now, he's still kind of a jerk. It was gradual and believable, and it's clear that not everyone was fully convinced of his Heel-Face Turn originally. Same thing with Sunset—while many believe that her redemption in the first movie was rushed, the other two movies really substantiated her struggles a lot more and showed that most people didn't forgive her right away.
But Starlight's redemption kind of made me cringe. Her, uh, Freudian Excuse didn't click as something that would equate to literally brainwashing ponies for her ideal utopia. It was disproportionate retribution for a single event that happened during her childhood, and in general the reveal of her "tragic backstory" was poorly presented. Furthermore, the redemption itself was also rushed—she literally changed her mind after a brief speech from Twilight, and it took—I kid you not—two seconds for her village, the people she wronged most, to forgive her. Granted, it was during a montage, but still—there could have been an episode on this that would develop the forgiving gradually. Afterwards, she was immediately accepted by pretty much everyone right at the start, unlike Discord and Sunset. Starlight Glimmer's redemption was one of my biggest gripes with the Season 5 finale and a major reason as to why I disliked it.
Basically, I don't mind villains being reformed as long as it's believable, the redeemed in question actually atones for their mistakes and receives the consequences, and it's a long and gradual redemption. I certainly wouldn't object to an episode about realizing that not everyone can change for the better, though.