It's not stupidity. Dilbert is just not used to the kind of dishonesty management expects from him.
In the formative years at school, brainy kids like Dilbert never had to cheat. People like PHB, on the other hand, had to cheat and bribe massively in order to get even those pitiful degrees in art or whatever they have. This lead to two opposing cultures among adults and an endless series of misunderstandings.
The strip makes Dilbert seem kind of stupid which I didn't really like (Dilbert and Wally are after all role models :)). I understand the satire here is directed at useless company policies and not poor Dilbert though.