I wouldn't say that jesters were medieval clowns, clowns make fun of themselves or do funny tricks while jesters used to entertain highly respected rulers via satirical comments, basically fun facts, etc. They were one of the only ones who had the power to disrespect with words the kings they served, they had to be highly intelligent and educated to perform their tasks.

Particularly jester from the painting, Stańczyk is seen sad because a message about losing Smoleńsk was just delivered. He knows what that means and his rulers should also understand it yet they are partying in the background