From the design of the movie to about 50% of the writing it's faithful to the source material, the other half is completely forgettable if you're older than 8 and even then you'd rapidly forget. The writers had the privileged position to spice up the childish tone of the book with the nonsensical mainstream humor of "Steven's Universe", without it feeling forced, but they didn't feel inspired as it seems.
Coming from someone who's first book series he could read on his own was Captain Underpants (explains a lot, doesn't it?), this is the film I wanted thirteen years ago and its tardiness is really my only complaint. In a whiz-bang 85 minutes, this film adaptation is one that that knows how to be a sweet parable on childhood, a great-looking animated film, and a charmingly silly commitment to its source material.