I haven’t read the comics. So I can’t tell you if it kept true to them. But as someone who knows nothing about the original Captain Underpants, here’s my two cents.
It’s a cute movie. Border line late night Cartoon Network type show, but it stays just clean and lighthearted enough to be kid friendly. There’s potty jokes, and even comments making fun of how many potty jokes there are. But in the end, the movie has a nice message, and the characters all grow and learn.
So yeah, overall I give it a thumbs up. If you have a kid, (boys will like it more than girls due to the literal toilet jokes) then I’d say take them out to go see it.