Yeah. TBH, I've had a feeling that we were, at some point, going to get a huge reveal that she is somehow related to the Luke, Leia or someone else that we know. Of course, the movie leaves us hanging, but that's not the only plot point with which it's done. I think she's only a Mary Sue if she doesn't have a good reason to be this powerful. I hope this does come up in the later movies, because it really seems like it should.
As for the lightsaber fight, not only was Kylo Ren shot (this probably interfered with his Force powers more than with fencing skill), but he was obviously a crappy fencer with a weapon unsuited for him. If you look at the scene, he's swinging that lightsaber like a club, trying to get Rey through brute force (something he's lacking at the moment). His weapon resembles a claymore, which can be fenced with, but this requires skill that he obviously doesn't have.