I think you missed the biggest thing which was her using large amounts of her money in British politics to push anti Trans legislation and court rulings. Because of that, it becomes clear that any money given to her is funding anti Trans activism. If you disagree with her, that is a pretty good reason to try to get people to not spend money on her products.
That being said, being grumpy about someone cosplaying a character seems odd because that (likely) isn't putting any money in Rowling's pocket.