Good Show Sir is the interesting blog which collects all the worst science fiction book covers out there. The first time I looked at this blog, I was amused to see the latest post featured a book I myself had.
Lately though they posted a Kurt Vonnegut cover. I love Vonnegut, but this cover is indeed bad. (Though I've seen worse on the like of Battlefield Earth.) Mostly, I feel like there should be a category called When Bad Covers Happen to Good Authors.
One of the things I don't think people realize is that writers do not necessarily get a lot of say over covers, especially if they're getting published with a big press. Sometimes writers will get to pick one cover out of a few acceptable choices, but that's it.
Don't despair for Vonnegut: his cover for Jailbird actually looks really nice.