Yeah, "Mary Sue" is a terrible term. It doesn't even consistently mean anything except "character I dislike". Vriska certainly isn't anything like the uninteresting good-at-everything characters of the fanfics that the term originates in. She's basically the only thing the fandom is talking about these days.
I can kind of see where you're coming from, though. She's successfully forced herself into the center of all attention keeps doing terrible things, and she's in desperate need of a karmic slap to the face. Getting beaten to death by Aradia certainly wasn't comeuppance. It was more like her second chance. Aradia's insane troll way of forgiving her, happening right at the same time when Doc Scratch lifted his alleged luck curse. And as second chances went, it was pretty sweet. She got her arm and eye back, and was learning the True Meaning of Friendship from being forced to cooperate with the other trolls to win the game. She was probably doing okay, right until she decided to create Bec Noir.
And Vriska
is doomed to fail. I don't think the other trolls (and humans) are going to be putting up with these hijinks indefinitely, so Vriska basically has the choice of killing everyone else, dying herself, or genuinely becoming a better person. At least if she wants to stay in the story. I guess she could also quit while she's at the top and just leave to never be seen again, but somehow I don't think that's likely.
Also oh shit update.