you can have:
Stupid as heard in 'Clever Polly and the Stupid Wolf'
Thick
Dense
Dumb
However these can be thought to be quite offensive.
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An antonym means opposite so the opposite of clever is dumb or stupid and the like.
No. Clever is an adjective. The corresponding adverb is cleverly.
cleverest
cleverer
This word does not have an opposite.