I don’t care for the term Mary Sue because it unnecessarily inserts gender into a conversation that can apply equally to male and female characters alike. But I don’t think the criticism has to do with no explanation offered. I think it has to do with lazy writing, and having a character who can do seemingly anything and everything whenever convenience calls for it.
Once you have established such a character and such a story, simply going back into the script and inserting an explanation for all of their endless abilities – seems to not address the problem in the first place.
It’s only lazy writing if the explanations were mere after thoughts and not something integral to the character from her conception. There is no evidence to support our theories on how exactly Rey was written so for me all that matters is that her competency is explained and for the most part it is.