So why do people, er, get their undies in a bunch about Captain Underpants? I have a list of possible issues:
--disrespect for authority
--pranks and bad behavior
--deliberate use of misspelling
--complete and utter absurdity of the plots
To help me think about this, I read the latest, Captain Underpants and the Revolting Revenge of the Radioactive Robo-Boxers. I hadn't looked at any since about 4 books into the series (this is #10), and was struck by a couple of things. For one, the text seemed more substantial. Also, the segments which George and Harold "write" themselves (with the bad spelling and all) were very limited. And the story? Well, "absurd" does pretty well describe it. I can't say it was laugh-out-loud funny for me, but I can definitely see the appeal to the kids.
So the issues:
Potty humor. Show me a little kid who doesn't love potty humor, and I'll show you a robot from the planet Dullness. Seriously, if parents think their kids are learning potty humor from these books, they are deluded.
Disrespect for Authority: See above. In fact, despite the boys being constantly in trouble, I don't find their behavior extreme (it was worst in the earliest books). And they DO make the principal into a super-hero. In many of the books, the boys do engage in pranks that we as parents our kids will not emulate. I think that most kids, though they will laugh at the pranks, also understand the consequences (and Harold and George do suffer consequences for most of their pranks). Oddly, there are no pranks in this book, only a desperate battle to save the universe with the boys working alongside Captain Underpants.
Deliberate use of misspelling. This is actually limited to the "comic book" sections that the boys write themselves, which seems to me to provide readers with a) easy recognition that this is not the writing to emulate, and b) a chance to feel superior because frankly, nearly all the kids reading this can do better than that. It's part of what's funny. The bulk of the narration is fine.
Complete and utter absurdity. I don't think this even deserves a response. I like absurd. Certainly it is so far over the top that there is no worry that any kids are going to mistake it for reality. And the claim that we now know what killed the dinosaurs is pretty funny.
Oh, and I gather some parents claimed "nudity" was an issue. I guess that's because Captain Underpants runs around in a pair of tighty-whiteys and a cape. If a cartoon drawing of a rather stylized human in underwear is their idea of nudity, I hope these people never go to the beach.
My bottom line: when kids are making the at times difficult transition from reading to learn to read to reading for pleasure, if some potty humor and laughter at the expense of an authority figure gets them to read, go for it. And for some suggestions on how to deal with various issues you may have with the books, check out this piece from NPR. My biggest issue with the current addition to the series is that it ends in a blatant advertisement for the next book, which I thought was a bit tacky. As opposed to tasteless, which defines the whole series, but in a good way.