No no, you’re supposed to love starlight unconditionally remember? She had a totally sad backstory that completely explains starting a cult and almost destroying the spacetime continuum.
Seriously, though, she’s the team’s pet in the same way Trixie or Luna is the fandom’s pet. They can do no wrong even if in reality their life choices are crap.
Luna fans: Luna becoming a nightmare, trying to kill her sister and become a tyrant because ponies liked the daytime better? Who cares, she’s perfect. I really relate to her. My social awkwardness and sense of being an outcast is totally reflected in her basically getting possessed and trying to commit sororicide.
Trixie fans: a self centered second rate stage magician who (sometimes unintentionally) caused harm to others to feed her egotrip? Nah, its endearing, not weird.
The showrunners: starlight’s totally the bestest character ever, right? The way she constantly resorts to mind control and other potentially harmful magical solutions to all her problems even after her reformation is just so quirky isn’t it? And that destroying time stuff? Well, a childhood friend moving away to go to school is totally sad enough to justify that. Heck, I bet we could justify straight up warcrimes if the perpetrator’s friend moved away when they were a kid.
These pet characters don’t have to be well written, appear often(in Luna’s case) or even have a well thought out and sympathetic backstory. Those who love them will either overlook or explain away how their actions are completely disproportional to their motivations. There’s nothing wrong with this per se, there is something wrong when the character is the pet of the showrunners. This means the team won’t ever question the quality of their fleshing out or the placement of this character. This means that in S7 when starlight was shoehorned in everywhere, while we saw a character spammed in a few too many places, all the team could see was a chance to put their favorite character on screen. Even they must have realized this was bad since they toned it down later on.
Taking a step back, the team pretty much let every reformed villain get away with everything they did. But, to be fair, no villain that got reformed really did anything as dangerous as what Starlight did and none of them became Twilight’s personal student right after they became good.