Don't worry your sweet little head. I think Rey is going to get rekt in episode 8.
Also if you look at it a bit more deeply Rey did struggle a bit in episode 7. Wanting to run away, lashing out in anger, bumping and banging the falcon when she first flew, missing blaster shots etc.
The "Mary Sue" argument is very weak imo
She's plenty flawed. She's unwilling to let go of the past and she's super judgmental.
Coming out of the theater I didn't dislike Rey. I just didn't feel particularly invested in her. By contrast, I thought Fen was more interesting, despite my questions about how he had sprouted, in a vaccuum, a conscience and a personality when no other Stormtroopers seem to.
Coming out of the theater I also had never heard the term "Mary Sue", though I certainly learned it very quickly. I'm not sure I understand the full implications of throwing around such a term, so I won't, but suffice it to say I think what makes a character interesting is their trajectory. Where do they start out, and where do they end up?
Rey's path seems to just sort of be
ascending. She embraces a destiny after an initial bout of reluctance. ¯\_(ツ
_/¯
Again, I don't dislike Rey. I like Rey. She's just not as exciting as she could have been. A more interesting Rey would have been someone we see as a bit rougher at the outset, a bit more of a survivor, a ruffian.. Maybe she's involved in petty theft to get by -- You know, something like that. That way it's much more exciting and fulfilling when she does "ascend".
Don't dislike her; Only dislike the missed opportunity.