See, I think if she were pure Mary Sue in TFA, she wouldn’t have had any fans after that film, nor would the film have had the legs to carry it through to such success at the box office *and* with the supposedly stingy fan-base. But since the film *and* the character were a hit far more often than not, and since TFA went out of its way to give her multiple extenuating circumstances for beating Kylo *with help*... I don’t think she can be qualified that way in TFA.
On the other-hand, I think if she were a true Mary Sue in TLJ, she wouldn’t have seen so many of her TFA fans get disgusted and turn on the Trilogy - because far more than “Mary Sue”, “Manic Pixie Dream Girl” fits her when she interacts with Kylo.
Rey’s actually a pretty sucky “power-fantasy” character in TLJ - at least if you want to have your power fantasy go through her. If you’re a fan of Adam Driver’s wobbly lip and haircut, though? And if you don’t mind Kylo not getting a clear cut victory over her in exchange for using her to make himself Superme Leader with what amounts to a complete lack of effort on his part? Then yeah, it’s a power-fantasy - for Kylo.
That needs to be reckoned with - the film where Rey was clearly the central character and Kylo wasn’t offered any special treatment by her or the other non-related characters was a success... and *did* see her struggle to pull out the unlikely dark horse victory as presented by the script and acting.
But the film that, to many people, solidified her as a “Mary Sue” actually *isn’t* interested in her, and in fact acts just as much as a power fantasy for Kylo as it does for her - albeit in a lazy, entitled way where he can fail upwards because everyone is supposed to sympathetic and identify with him.
TROS, yes, is another kettle of fish, but has to balance the blatant Kylo-favoritism of TLJ it’s the goal of trying to make Rey the main character again. Trying to make the actual protagonist match the Creator’s Pet from the last film is just a bad place to start.
Seriously: Rey’s problems had far less to do with LFL trying to push an agenda with her or trying to make her a star, and far more to do with LFL (and Rian Johnson in particular) expecting, treating, and trying to push Kylo as the star of the ST, which led to Rey being broken in a story that would not allow her to act naturally and used her as a handy Deus Ex Machina instead of as a protagonist.
And unfortunately, a bunch of the assholes who whined and moaned about a woman and black guy being the main star *before* TFA was even released stuck around, and shaped some criticism after the ST was over to still put more blame on Rey or Finn for its more lackluster elements, rather than on the jarring and obvious way things changed around Kylo.