While in British Englisch burnt clay is considered as identifier for grades of stupidity (to be as thick as a brick), American English chooses a more calorie laden option and defines a person with supposedly very low IQ as “as dumb as doughnuts”

It being the staple food of US law enforcement officers is pure coincidence and does not mean to ridicule their mental abilities.

z.B.:

This chick sure is hot, but frankly, she’s as dumb as doughnuts.