I think Rey can still want revenge against Palpatine without her parents needing to be noble. Firstly, they were still her parents, and despite the level of abuse/neglect implied by literally selling her into slavery, she’d still probably be opposed to them being murdered, especially since she doesn’t remember them well enough to harbour any personal grudge. Secondly, he’s Palpatine: he’s responsible for a huge amount of death, for turning Ben and starting the First Order (and thus getting Han, Luke and Leia killed), and is actively trying to kill the rest of her friends. Difficult guy to not want some kind of revenge on.
Why? The point of TLJ is that she doesn’t care about her parents anymore and now has to stick with the Resistance for validation, because she wasted all those years for nothing caring about awful, drunken parents who neglected her. She’d want them to suffer for neglecting her and believing she was nothing.
Having her go back to caring about them regresses that.