It’s not the term itself, it’s how it’s used. And it’s sexist because no one ever accuses male characters of being a Mary Sue, or a Gary Stu, or Harry Flu, or whatever.
Right. Obviously the author of that serial didn’t mean for it to be an offensive term. So the “a woman invented it so it’s not offensive” argument, in this case, is stupid. For the author, it was the name of a character in a satirical story that made fun of outlandish fiction by creating a hyperbolic example, who happened to be a girl named Mary Sue, and now society has turned the name into a one-sided (gender-wise) concept that is offensive, as is how a great deal of offensive terms “originate.”