It's not necessarily about not liking Rey, it can about not liking how she's been handled.
I like most of the new characters Rey/Finn/Poe, but I think their potential has been mostly botched in the first two films. Especially Finn.
Right. Finn's character had so much potential, but he's been completely wasted. He was one of the characters that felt completely new and unique. He was taken as a child slave into the First Order, but then on Canto Bight, he's not the one trying to go out of the way to free the child slaves. Such an opportunity there, but just wasted.
Rey is written with some flaws, but there is no character that challenges her on those flaws and those flaws never lead to her actually losing anything in the end. Take something as simple and subtle as the 2 scenes where Luke and Rey are in the Falcon shooting down TIE fighters. When Luke gets one and Han is sensing his over-confidence and naivety, he tells Luke to not get cocky to keep him focused on the mission. Rey on the other hand makes her comment on liking it and then proceeds to get a triple kill and pulls all the TIE fighters away from the fight. At a time when her over-confidence and naivety should be put in check, it's just fed more and is led to more success.
Poe actually got his flaws put in check by Holdo. Whether you like Holdo and how that arc was done or not, at least there was some decent character development with Poe.