In my experience, at this point in fandom Mary Sue is a largely meaningless term used to denigrate women who get above ~their place~: most popularly in most fandom circles, essentially when they're celebrated for or confident about the talents they possess. Kind of like 'slut,' which often has nothing at all to do with actual perceived sexual activity. Maybe at one point it described a valid phenomenon, but an inherently gendered slur is problematic from the get-go, especially when the problem is ostensibly a writing style that hasn't matured or an advantaged character without depth.

Since Blossom may be advantaged, but an advantaged human with a great deal of textually acknowledged flaws and foibles and multiple character foils she does not agree and interacts organically with - a friend of mine suggested, and I agree, that the most consistent feature and true flaw of the character type originally described as a 'Mary Sue' has in every example I've seen boiled down to a contempt or unkindness towards other characters that is never called out - it pretty much sounds like readers trying to put her ~~in her place.~~ Whether they realize it consciously or not, many consumers with misogyny, internalized or otherwise, are made uncomfortable by fiction that thinks ladies are rad as hell and also human, and because it makes them uncomfortable they find hostile catchphrases for it.

But then, my essays on why I loathe the term 'Mary Sue' in fandom today can be left for some other time. And I'm pretty much exactly where you are when it comes to the writing and mental places, UGH. :(