Yeah, it's one of the things that really aggravates me about the haters of The Last Jedi. They vaunt the original trilogy, but I doubt any of them have examined it with a critical eye.
If Rey's a Mary Sue, then so is Luke. His Force training was the equivalent of a long weekend at a Hyatt Inn, and it's shown many, many times in canon that he was kind of a cruddy instructor, mostly because he'd had so little instruction himself. Luke only had to stub his toe and Force Spirits would pop up to advise him.
Kylo Ren is pretty much symbolic of the dark side of the first 6 movies, where it went from heroes coming from anywhere (remember the dozens, hundreds of diverse Jedi in the prequels?), any race, any gender, to just the Skywalker (white) men (Anakin, Luke, and Kylo/Ben). Kylo comes from a pure and privileged line, he had training offered pretty much from birth and when he left, there was someone waiting to offer more training and praise.
The whole point of The Last Jedi was that heroes could come from anywhere (which derailed J.J. Abrams original plan to tie Rey to the Skywalker or Kenobi line, and placate the grouchier "fans"), because The Last Jedi is the darkest hour, scrambling for any hope left, and a hero has to come from somewhere.
I think the reason the haters are bagging so hard is the same reason straight white middle-aged men hated the Twilight movies: Because it wasn't explicitly for them.