I myself have asked a question regarding this, but over some time I have come to see Mary Sue as an bad term, a term that is used to bully authors into creating generic, standardized characters. I'll explain what I mean below.
The term Mary Sue has been very harmful to women characters. Anyone who tried to create a strong female character was immediately mocked as a Mary Sue:
"... fear of creating a "Mary Sue" may be restricting and even
silencing some writers. Smith interviewed a panel of female authors
who say they do not include female characters in their stories at all.
She quoted one as saying "Every time I've tried to put a woman in any
story I've ever written, everyone immediately says, this is a Mary
Sue." Smith also pointed out that "Participants in a panel discussion
in January 1990 noted with growing dismay that any female character
created within the community is damned with the term Mary Sue."
This has led to the situation where people don't create strong characters at all. All characters follow the same cookie cutter, Disney/Hollywood formula- they will start off as comically bad- ignoring their children, mistreating their spouse/ employees etc, and by the end of the movie, suddenly becoming goody good perfect human beings, having learnt all the cliched lessons the script had.
I have written before about how I have stopped reading crime fiction, as almost every detective has a marriage problem, is an alcoholic, and usually is a jerk. I used to wonder why this was, till I saw all these online forums, where new writers are asked to give their characters "flaws", which the characters can overcome. And so a "flaw" is bolted on the character, which they can then "overcome", and hence tick the box that says "character development".
And that is the problem of the Mary Sue tag- writers are scared to write the type of characters they want, but have to think about what others will think. Even reviewers are caught up in this- I read an enjoyable fantasy/comedy book in which the hero defeats a bunch of demons. And most of the negative reviews basically said "There were no character flaws." How dare you write a book where the hero wasn't abused as a kid, didn't have his parents murdered, isn't suffering from alcoholism, and doesn't hate his boss?
So I'm ranting a bit, but the term Mary Sue has harmed writers a lot, and should not be used.