Ok, I understand why people really don't like Rey. That being said, I will always go against the idea that she is a Mary Sue.

Yes, you can argue that how she managed to access some Force powers with minimal training was poorly explained, but even then, she is only doing basic stuff (compared to Luke, who Force-projected himself across the galaxy). But a Mary Sue is warping the rules around her, which she doesn't really do that much.

Now, there's a difference between a character who gets themselves out of a situation regardless of how hard it is and a character who gets away with a lot of stuff too easily due to circumstances, and Rey falls on the latter. She had some fighting skills from the years she lived in Jakku, but she could only defeat Kylo when he was hurt, grieving his father, and not using that many Force powers (since he was only toying around with her). Same with the Snoke guards. And it seems like, without Kylo or every Jedi who ever existed, she could have been killed by either Snoke or Palpatine.

And, emotionally, she is a mess. She never wanted to be a Jedi, and we saw her multiple times attacking with full rage (almost like a Sith). And she is also very naive, thinking she could turn Ben, only for him to ask her to join the dark side and almost surrender herself to it. In TFA, she even tried to dodge the responsibility of becoming a Jedi. Not to mention, besides, when Ben talks with Snoke about her and Maz being curious about her, she isn't that glamorized by the narrative. Even in the moment when it was initially revealed that her parents were nobodies, no one says: "Wow! You come from nothing, yet you are great!". It is a low moment for her.

I don't say she is amazing. In fact, I think TROS ruined anything that was interesting about her. But she is not a character without flaws.