If she wasn't a skywalker, than that would make her even more of a Mary Sue than not, as some people are worried.
Like...her being a Skywalker explains why she's naturally a good pilot and fast at picking up the Force. If she wasn't, if she was some rando, then she's a Mary Sue.
Even if she WEREN'T a Skywalker, that doesn't mean she's not just extremely attuned with the force. The Skywalkers aren't some mythological gods, they're mortal beings, just like anyone else. There were a ton of individuals who were nearly if not as strong as them. Palpatine isn't and look how powerful he was. Or just about anyone in the PT. Yoda's not. Windu is not. No one on the council was (before Anakin was partially admitted). Obi-Wan ultimately defeated Anakin (he beat him in Ep. 3 and sacrificed himself in 4) and he's not.
This argument doesn't hold any water.
I personally believe she IS a Skywalker and that she's Luke's but even if she's not, that doesn't make her any more of a "Mary Sue". That's just a ridiculous argument that wouldn't have been an issue if she weren't a woman, honestly. And I'm not even one to really delve into those types of arguments.
Anyway, like others have said, it's not even worth it and there's a whole thread dedicated to this non-issue.
I really wanna know when they start shooting dammit.