i think deleuze says that the question of "how there can be stupidity" is a "transcendental question". i _assume_ because it is not a stupid one and so guarantees that there is something more stupid [rather than it necessarily being a stupid question!].
but the question may foreground our own stupidity. in effect then, the question would be something _like_ how can the answer to the meaning of stupidity, be_stupid.
at least assuming that questions only foreground their answers. for the sake of the exercise, let's suppose so.