Kim Possible's ending was fine. Sure, it wasn't the best, but it was still passable considering it was supposed to end an entire season beforehand. At the end of the day, it was still satisfying enough to leave you with a sense of closure.
Star vs on the other hand is just plain inexcusable in every way. It's ending is nonsensical AT BEST. It contradicts virtually everything the series has built up, even in the same goddamn season.
What was the point of building up Moon's character to be more lax about political matters and putting her faith in Star if she's just going to throw all that away and instigate a war she was against the entire series? What was the point of dedicating THREE filler episodes to the wacky misadventures of Star's sentient spells if you were just going to kill them all off and expect people to not feel sympathy over it? Why was Eclipsa such an insufferable Mary Sue? What was the point of showing the end result of Cleaved being the literal apocalypse while treating it like a good thing? How the hell do the clearly magical beings like Lazer Puppies and Ponihead survive while the other magical beings like Doop Doop and the Unicorns just fucking die?
Star takes it by a landslide. KP's wasn't great, but at least theirs was coherent and tried to be satisfying.