Let's be a bit more succinct, shall we?
The original Mary Sue–written by a woman–was a parody of wildly unrealistically important characters that are obvious inserts for the author. I think the idea was to try to goad fanfic authors into being more creative.
Way too many people, mostly dudes of course, use Mary Sue as a way of saying, "waaaaah, the protagonist is a feeeeeeemale." Waaah, Rey is too perfect! Oh? Luke goes from farmboy to hero of the Rebellion in a few hours' time. Cry some more, idiots.
And yes, I get the appeal of trashing superheroes as mere male power fantasies, but let's think about a couple of them for a second. The first is Superman. Or, more to the point, Kal-El. Created by Jerry Siegel. Kal-El can be translated to mean, "Voice of God," based on interpretation from Hebrew. How about a Marvel character, Spider-Man? Peter Parker. Written by a guy named Stan Lieber that the world knows as Stan Lee. Parker's from a predominantly Jewish neighborhood.
Things were different in Siegel and Lee's young adulthood. Hell, even within my own lifetime, Jewish people haven't always had white status. One of the most Mary Sue'est of story lines of Superman is surely Superman defeating the Nazis.
I can't think of any reason why Jewish people would have wanted power fantasies back then, yeah?