Oh, Captain Underpants. You know, these are still very entertaining sometimes, especially when the text itself is aware of its ridiculousness ("and before you could say 'convoluted plotline,' it disappeared into the noontime haze"). And I like that it keeps up the trend of throwing in advanced vocabulary words that can be sort of understood by younger kids because they're in context. But there were a few things that kept me from enjoying this one as much as I liked the others.
One, I liked them better when each adventure was self-contained and I don't have to remember things like where those eggs came from and who tried to destroy the town when (which becomes more important when you're dealing with silly time-travel devices). Two, some of the random crap that happens felt more like random crap happening than an overall story that happened to be silly. And three, two aspects made me sort of uncomfortable: A several-chapter scenario in which teachers stripped off their clothes and ran around in their underwear, and repeated references to "the goof house" (the book's term for an institution for mentally ill people).
The teachers' partial nudity happened because Harold and George convinced them all they were dreaming (and apparently if you're dreaming, the thing you long to do is remove your clothes immediately and cavort with other teachers, playing slip-n-slide in the school). This is hard to explain, but I'll try: First off, it's pretty common for kids to mock their teachers for being fat or ugly, and the concept of seeing them partially naked is usually expressed like "ewwwwww ugh fat ugly gross old people, DISGUSTING!!" In other words, it's like portrayed as self-evident that adults' bodies are gross if they're not models, especially if they're overweight, which is used as shorthand for making a character whose body inspires a revolted reaction from someone else. I'm pretty sure every teacher running around in underwear was drawn fat, and the children (as well as the reader) are supposed to be disgusted. I think it was a poor choice.
The "lol something traumatizing happened to someone and they got shipped off to the nuthouse" trope is used as a joke in a LOT of stories, and that's also kind of awful, especially since they trivialized it by calling it the goof house. I know it's just a silly cartoon book, but people really do get institutionalized over trauma or mental illness, and if it happened in your family or to your friend and a book made light of it, that would be difficult to explain or accept. It's a message I'd like to see way less of in cartoons and kids' books.
However, the book also had some of the stuff I always loved about Captain Underpants--the silly flip-o-rama violence, the ridiculous adults, the relationship between Harold and George as school mischief-makers and best friends, and of course Captain Underpants himself. My favorite bit was the middle of the book when Harold and George create time clones of themselves and then have to deal with interacting with each other, taking tests, and managing the reality of having extra kids to feed while hiding this from their parents. The callbacks to other books were also sort of funny, like the return of the Turbo Toilet and Melvin Sneedly acquiring superpowers from Mr. Krupp's toenail through a complex process of DNA extraction.