JKR is one of the few writers/actors/musicians who, no matter what they do, will sell a shedload of books, albums, whatever anyway. If she'd released a book with 500 blank pages then she would get 1 star and 5 star reviews in equal measure. Same with Coldplay, U2, Dan Brown and other big hitters.
So, really do all these bad reviews make a difference too her? The cash tills will still keep-a-ringing.