My kids loved Captain Underpants!
Of course, I think farts are funny, so I'm probably wharped. My kids are bright, love school, read at a college level, blah, blah, blah....they still found Captain Underpants funny. It's goofy, it's over the top, and there are a lot of plays on words and language in it. It's fairly clever, and I think it's great commentary on some of the negative aspects in our culture (like the authoritarianism of some institutions).
Yes, it's potty-humor. So what? Do you never snicker at a crass joke? Do you only read haute literature and watch Merchant Ivory films? No, of course not. This is the same deal.
Ironically enough, the humor and the anti-authoritarianism of the novels is at least in part based on the idea that potty-humor is unacceptable. Most adults and the bad-guys in the novels don't approve.
Do you let your daughter read Roald Dahl? While there isn't much potty humor, the adults, school, and other institutions rarely come off in a good light. This is the same deal, just with a giant, nuclear toilet. Yeah, it's sort of like G-rated South Park meets Roald Dahl. Absurd, witty, goofy, and bathroom talk.
I vote you let your daughter read them. My kids read some real trash, but I wouldn't even place this in real trash, because I honestly think they are fairly well written graphic novels. Trash is the stupid stuff like "The Babysitters Club (or whatever it's called)". Which my oldest loved for about 2 weeks one summer. Both the girls occasionally enjoy reading something totally worthless, but then they go back to their Jane Austen, Brian Jacques, Sir Arthur Conan Doyle, etc.
I mean, come on, I've read all the Twilight books and the Sookie Stackhouse novels. My head didn't implode. I didn't become some faintly daisy or a blathering idiot. My IQ didn't plummet. It's okay to let them read books that aren't on the harvard summer reading list! Reading is fun!