I'll put my argument very succinctly-no, she is not. She is not perfect at everything, nor is she an author's avatar. She has fears, limits and insecurities. She is plagued with self-doubt, and a struggle to find where she belongs.
No, she isn't better at everything than everyone else. She beats Kylo when he is wounded by a bowcaster that blows an armored stormtrooper back.
She is a trained survivalist who has a working knowledge of ships and parts. Yes, that translates in to fighting with a different weapon. Weapon's training is weird like that.
No, she isn't a better mechanic than Han. She just knows what part is causing the issue.
Finally, if she is a Mary Sue, then so is Luke and Anakin. So, neither new, nor offensive to Star Wars if we are equally applying the criteria.