OK, Rowling isn't a terrible writer then. I'm only basing my opinion on 1.5 books (yes, I gave up halfway through 2) but she is distinctly average. Kids like her. Fair enough, really. What annoys me about her (from a writer's perspective) is not her, but the people who assume that all writers live like this and the "ooooh, going to be the next JKR then?" type comments that ensue if you mention you are working on a children's book.
It's also very irritating when you have to hustle, hassle and prostitute yourself to get people to go and seek out your own books in shops. And the fact that they constantly go on about JKR as if no other books existed.
It's a bit like being a footballer in a decent second-league team and finding every social event dominated by discussion of David Beckham, and that people - even your friends - are queueing up to see his games when they won't come and see yours - it's not that you begrudge a talented/lucky person their success, just that you wish a bit of it could be spread around.