That's cute, but Mary Sue is 100% a bullshit derogatory term for female characters used by men (using that term loosely) who are angry because said female character is powerful or successful/etc. Nothing more, nothing less. Y'all can pretend this isn't a thing all you want, but it is very much a thing.
"omg Rey is a Mary Sue how is she so powerful REEEEEEEEEE"
"Captain Marvel is such a Mary Sue p.s. omg smile more REEEEEEEE"
"Sylvanas is a Mary Sue who cares if she is actually deeply flawed and we understand where her new power comes from REEEEEEE"
I could go on and on, but it is genuinely hilarious to me how mad some guys get at the idea of a fictional female character that is powerful or accomplishes something. Because pretend characters with pretend abilities in a pretend storyline are so important, right? Its totally worth stroking out over Captain Marvel, or Rey's use of the force, or Jaina/Sylvanas/whatever.
And literally nothing about Sylvanas is 'flawless' - she's probably the most deeply flawed, hateful character in the lore at this point. But sure, lets call her a Mary Sue because reasons™.
Oh hey, I found a photo of people who care what you think.