This is a “simile”, a phrase of the form “as X as Y”, where two things, X and Y, are compared, often in a fun way. The X part is what is important and the Y part is a just memorable comparison, usually starting with the same letter as X but having little to do with the rest of the sentence. Many have references to animals or ancient people:
He couldn’t find his glasses because, without them, he was as blind as a bat.
She never stopped doing things and was always as busy as a bee.
After winning the lottery he was as rich as Croesus. (a very rich king from ancient Turkey).
John was usually a naughty child, but today he had been as good as gold.
The old man did strange things and everyone thought he was as mad as a hatter. (people who used to make hats often worked with mercury, which affected their behaviour)