Are all forms of ism necessarily a bad thing?

A number of feminists have mentioned the fact that there are a large number of ist books out there, one particular that was mentioned was the Twilight series being ist towards women through a number of 'traditional' aspects of it. But is this form of ism really detrimental in any way? I mean, most of the readers are under 20 year old girls that have been bought up in a post-feminist generation, and from what I've observed, the only harm it's done to any of them is the fact that it has made them crazy over it. That said, how can we expect all literature to meet the feminist criteria for ism? Few books that happen to be suffused with political correctness ever get on the best seller lists.