These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Designated Hero: George and Harold have a tendency to slip into this with some of their pranks. In the first book, they cause enough disruption at a school football game to get the school to forfeit it, and subsequently ruin everyone else's day, and Captain Underpants is later created as a result of their attempt to avoid being punished for this. In the second book, they sabotage the school's Invention Convention out of spite, due to them not being allowed to attend following the prank they pulled last year. And in exchange for fixing all the damage done by the villains, they have Mr. Krupp cancel their punishment and let them run the school for the day, during which they sell a lot of the staff's furniture to pay for the party they throw for all of the students. Granted, they do try and stop the villains in every book, but the fact that many of them (the Talking Toilets, Poopypants, Wedgie Woman) appear as a result of George and Harold's actions doesn't help their case.
Granted, there were unordinary circumstances. Poopypants/Tinkletrousers was eventually going to crack and the fact it happened at Jerome Horwitz kinda puts it in debate. With Wedgie Woman, they were unaware of the fact that the Hypno-Ring has reverse effects on women.
These only apply to the first five books. As the books continue, we see more outside threats not by them or by returning villains (the latest book bringing back the Turbo Toliet 2000.)
In Wrath of the Wicked Wedgie Woman, it's revealed that the ring was recalled because for unknown reasons, it worked the opposite of its wielder's intentions on women. Unfortunate Implications aside, this might seem like jumping the gun to younger readers, but George and Harold used it on their already-Sadist Teacher and look what happened. Now, what about all the other kids who had one? Chances are, the first thing a lot of them would use it on is their parents, who they typically have a very low opinion of. All of a sudden, there'd be an outbreak of Abusive Mothers all across the country because the kids demand that their every whim be catered to. That wouldn't go unnoticed and would be a pretty damn good reason to recall them.
Jerkass Woobie: Professor Poopypants. He just wanted people to stop laughing at his name, though George and Harold point out that he could have simply changed it rather than forcing everyone else to change their name into something ridiculous.
Nausea Fuel: Inevitable in Toilet Humour-themed series, but the Bionic Booger Boy gets special mention. His descriptive introduction is cut off when George yells at the narrator for making everyone sick.
The Bad Future created by Tippy Tinkletrousers. On top of this disaster-layer cake, we also saw gigantic zombie nerd-versions of George and Harold. Also from this section of the book, Tippy is crushed, blood and all. Doubles as a Tear Jerker.
The Scrappy: Melvin and some of the adults at Jerome Horwitz Elementary are this to a lot of fans.
Taken Up to Eleven in (appropriately enough) Book 11, where the boys convince all of the teachers and faculty that they're in a dream. This prompts the grownups to release their inhibitions by performing all sorts of shenanigans in their underwear. And yes, Dav Pilkey illustrates this repeatedly...
Tear Jerker: The ending of the 9th book was REALLY sad.
Book nine reveals that Harold's father walked out on the family when he was six, and Harold created the Captain Underpants comics as a coping mechanism. Mr. Krupp in his Captain Underpants identity is a kind of Bumbling Dad Parental Substitute for both of the boys, but particularly Harold...