Rey is a Mary Sue only if you consider that she does everything without any kind of training. So to believe this you have to actively ignore that all her knowledge about technology and ships comes from years living in Scrap World where it’s all there is to learn about. Then you have to also ignore that all her Force knowledge comes from things she’s heard (you can lift rocks) or witnesses (thanks to Kylo Ren). Then you also have to ignore that she “beat” him even though (A) he thinks a lightsaber is something to break TV screens with and (B) he was bleeding out/not trying to win.
The actual “awakening” is a bit iffy but I like the Yin-Yang idea. Then she goes and hands Kylo the First Order leadership on a plate because of her own naive beliefs which is a massively stupid thing to do, but she heard somewhere that people can be “turned” good. I kinda like Rey.