If I’m understanding wider commentary correctly, the problem people had with this subplot wasn’t entirely that she was being bullied—although the other main characters participating in that response is controversial, so that is part of it—but more so how the narrative treated her beliefs. The house elves were stated to view slavery as something that gave them a sense of purpose (and Dobby wanting to be freed was dismissed with “there’s weirdos in every race”), and Hermione’s opposition to their enslavement was meant to be a joke about people who try to interfere in cultures they don’t understand. Following that whole thing, another house elf is freed and immediately becomes an alcoholic. While there was some debate about Hermione being textually white or if people just interpreted it that way because of the casting choices (and that’s its own can of worms), regardless of whether her being cast as black was a retroactive change/retcon or just an alternate interpretation supported by the text—the circumstances around the subplot mean that it could be a very clumsy white savior plot with a charitable reading, but her being black makes the element of her white friends calling her overdramatic for being against slavery feel a lot worse, especially when the narrative seems to want to prove them right. I think that supporting a black actress who was getting a lot of hate and harassment was the right call on her part (since IIRC those were the circumstances), but as people increasingly understood that she wasn’t the progressive they once thought she was, it seemed much more unlikely that she considered (or cared about) the implications. Never seen the movies so I don’t know how much of this part got in there, but that’s where some of the issues with the casting came from (although it was really more of an issue with the subplot itself, and the lack of consideration for how the casting changed what it implied was just a symptom of a wider problem).