If this is the case, why they don't just say she is a boring character, instead of labeling her as a "Mary Sue"? In other word, why they don't use a gender-neutral adjective to indicate their distaste..?
Because people think it's funny a few sensitive people get put out of joint over a silly joking name. Especially when you go into the gender neutral crap which only a minority if people even care about.
People just need to lighten up.