A Mary Sue doesn’t cease to be a Mary Sue just 'cause she has ubermensch genes.
Yeah, but it helps. I guess it comes down to what one wishes to preserve most in the canon. Like I said, I hate the genetic thing and have hated it since the ridiculous Leia retcon in ROTJ. But I dislike the ‘no training required’ thing of the ST even more, because it takes away from Luke’s arc and struggles. I’d rather roll with a version of Force genetics (I find a Sith version of this at least mysterious enough to carry some weight) to justify Rey’s power levels than go with RJ’s random ‘X-Men Force powers’ thing.
Luke’s struggles are due to lack of faith and lack of concentration. Rey had heard of the Heroes of the OT. Rey knows what a Jedi can do from those stories. Then comes face to face with it in a way Luke never did (he didn’t see Vader do anything in canon until TESB during their duel and I don’t think he saw Ben do anything either). Ben gave him one lesson on the falcon and then he is able to make the shot that takes out the death star and then later is able to levitate his lightsaber before his second lesson with Yoda. Luke had doubts. Rey didn’t. Rey had belonging and abandonment issues, Luke never seemed to. He wanted to know more about his father, but he knew who he was and that was fine. Rey has plenty of struggles and failures in the films. More than Luke actually. No one powerful in the Force seems to have any problem using it once they see it used and Rey saw it used by Kylo many time. In fact, you could say that he activated the Dyad by trying to read her mind and that he basically taught her everything he knew and she just had to concentrate to draw on it. Snoke implied that he linked them, but I don’t think he realized what he was dealing with. They stayed linked after he died. So Mary Sue? Nope. She has too many flaws and failures to be one. Sure she picks up the force quickly, but that is not the only part of her character and she rarely does it right the first time.