Perhaps some clowns are so creepy because they are just the kind of caricature that makes the mind itch and ache by depicting the ugly side of a truth. Humans tend to be mischievous fools and pretenders. Suppose its quite unpleasant to be reminded of this sad fact, as one is as human as the next man.
Besides, some clown makeups inscribe features of a skull on the face. Suppose the subconscious perceives these features, especially if the clown is bald, although the round red nose and the thick hair around the shining plate obscures them. This secret reminder of mortality, which cannot be avoided because it is perhaps not even consciously noticed, might be one of the reasons why some clowns are so creepy and / or offensively bothersome.
