I started to realize how many similarities there are between them. Captain Underpants is a lot more similar to the first three seasons of South Park though. Both of them were considered tasteless and vulgar and moral guardians parents that didn't appreciate the vulgarity of them tried to get them banned. They're both about kids getting into situations that they don't really want to get into. And despite looking shallow from the outside, there's actually really good lessons hidden underneath all of it. Like for example, the 4th Captain Underpants book is about a scientist named Professor Poopypants, and because of that, when he became a substitute at George and Harold's school, he was immediately made fun of. Then he becomes a supervillain who tries to get the entire world to change their names to something ones that are just as stupid because they pushed him to the edge. On the outside, it sounds stupid (even the name of the book does), but the lesson is a really good one, which is that you shouldn't make fun of people because of their names. In the first Christmas episode of South Park (excluding the Spirit of Christmas shorts, because those are prototype episodes), because Kyle can't do anything on Christmas related since he's Jewish, he comes up with his own mascot for Christmas, Mr. Hankey, which is literally a talking piece of crap. No one else thinks he's real and he ends up getting in trouble because he just looks like a normal piece of crap to everyone else, but it turns out that Kyle didn't make him up, and he ends up being just what everyone in town needs, who are in shambles after they try to remove everything from Christmas that could possibly be considered offensive to anyone. The episode shows how hard it is to be a Jew on Christmas, like the name of the song Kyle sings, since he can't celebrate Christmas and feels left out. The lesson is also a stock aesop pretty basic moral about how Christmas is about family and everything you've ever heard in a Christmas special. The point is that even though it's all covered in crudeness (and in later South Park, offensiveness), there's a good reason to all of it, because it's trying to teach you a lesson. I think the people that wanted to ban either of them are probably people that didn't understand that.
As a last note, I think Captain Underpants as a whole is better than the first three seasons of South Park, but South Park as a whole is better than Captain Underpants. Also the Captain Underpants show on Netflix is pretty cool and if you like the books you should check it out.