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 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.
Yeah, but that just a way of bypassing the argument. The criticism is, that she is too perfect, and the progress in her knowledge of the Force too fast and easy, compared to earlier protagonists, and some of the better protagonists in fiction in general. I think those critics have a point, even if I don’t agree with everyone of them. I’ve defended Rey’s progression in TFA, but now feel her character has stalled in TLJ, while gaining Force powers exponentially.