I think most of them are averge. Here are most of the probable opinions on characters being dumb, and they are wrong.
Marker only sounds a little weird, but is still average.
David just says aw seriously, but he is capable of saying other things. He just likes to say his catchphrase.
Dora doesn't talk often, but she seems to be able to speak fine when translated from Pretend Spanish to English.
Roboty doesn't talk often, but he is average.
Teardrop doesn't talk, but she's surely average due to other interactions.
Pencil's intellect varies from being a hiprocritical generic highschool girl (below average), to average.
Match is just a bit smarter version of Pencil.
Fanny doesn't just bluntly hate everything, she does it because of a
wise opinio.
Sponge isn't a "stupid annoying bore" like Pencil says, Spongy is average.
Bottle is just childish, not dumb.
And finally, in BFB, Book was just really distressed about wanting a great team, away from Freesmart. She wasn't being dumb when she was behaving like
this, she was really stressed.