The terms Mary Sue and Gary Stu have no meaning anymore and are just blanket terms for characters people don't like (LEA)

Holy shit. If there is a character that it significantly powerful or talented you better belive that when they get criticised you'll see the term "Mary Sue" or "Gary Stu" as if that's an actual statement about their quality. Yuta from JJK, Midoriya from MHA and of course Rey (as if I even need to specify which one I'm talking about)

Notice that when you ask what makes them a Mary Sue they go in circles and just listen off the character's traits and on screen accomplishments and portray them in a bad light.

Honestly I think it comes down to people not liking the fact that whatever media they're consuming does not share their opinion on whatever character they are insulting. So when they see a character they don't like accomplish more and more it makes them even more angry