https://youtu.be/CALcI-4vM8E
This explains it very well, also points out another major star wars character as a Marty stu.. Phantom menace anakin.
I haven't seen nor heard a better explanation than this, nor a more balanced or reasonable one that covers the other bases (it not being sex is, daisy ridley doing a good job etc)
This is even before TLJ, Where the Mary Sue-Ness is just magnified.
I find myself wishing this new trilogy didn't exist, and I quite liked the prequels despite their flaws.
Post phantom menace the Marty - stu Ness of an akin was bearable, he had decent training and there were already 4 movies establishing he was very very powerful, and even then he was more fallable than Rey.
Anyway, just my two cents