The clown is not random. A good clown follows a logic, just not the one the room expected. A clown joke relies on a plausible inference. A man in a ski-mask carrying a comically sized bulging bag probably warrants attention. A dark cloud probably means rain.
But none of these are hard truths. What you see does not logically entail what you expect. And the clown uses that. It relies on our plausible reasoning, these tiny hidden assumptions, and treats them as if they were optional.
Plausibility as a concept does not make much sense to me, but nevertheless I will continue using it. It lives somewhere between a proof (in what formalism?) and a feeling.
Is the gap between plausibility and semantic entailment what might make humor funny?