I dislike the term "Mary Sue." Like, a lot.
Look, I understand that some people think that "X" female character is written poorly, and in the case of Rey in The Force Awakens, I would actually agree with you. But the term "Mary Sue" is literally meaningless when it's not used in reference to its original meaning. I look it up on TV Tropes, and it comes with 5 different questions challenging the label "Mary Sue," 14 different subcategories for the term, and 13 different applications for the term (that don't include the applications for its 14 different sub categories of the term). At this point the term "Mary Sue" means so many things that it ends up meaning nothing outside of the character simply being poorly written in some way. (In which case, it would be better to tell me that you think the character is poorly written!) As a storyteller myself, if people told me I had a "Mary Sue" character in my story I wouldn't even know what they're referring to. It's a meaningless label that confuses communication rather than clarifies communication.
As for Rey herself,
I do feel that the ending to The Force Awakens is rushed. But does anyone know why? Is Rey the poorly written character because she mysteriously overcomes the villain? If that's the case, wouldn't simply knowing about her mysterious backstory answer that mystery behind her abilities? If it does, well, it's still poor writing, but it's no longer what was considered the "Mary Sue," now is it? On the other hand, it could be that it was Kylo Ren who was poorly written, and not Rey. After all, it seems rather anti-climatic that a villain who could stop blaster beams in mid-air get bested by some noob. So the fault in character development could lie with Kylo, and not with Rey. (See what I mean by the "Mary Sue" being a meaningless title at the end of the day?)
That's all I have to say about that.