Note there are specific kinds of clowns in the real world. People may fear the auguste, the stereotypical clown who acts and dress the most flamboyantly. Whiteface clowns are unsettling because they are unexpectedly intelligent, even scheming. Character clowns - like Emmett Kelly's "Weary Willie" - by contrast have very muted designs, and can be difficult to identify by the TV audience. You rarely see them anymore.
Real world clowns clown: while clowning is intended to be funny, it can be received as ritualised (self) humiliation, and make modern audiences quite sick to the stomach.
That John Wayne Gacy, one of the most famous serial killers in the United States, worked part-time as a clown doesn't really help their case. You don't want your kids around the Monster Clown, but the character's parents seem oblivious to their kid's fears. The AP once used a stock photo of Gacy for National Clown day.
A 2007 University of Sheffield study found that both young and older children universally fear or dislike clown images. Now a lot of hospital wings for child patients will have to be repainted.(Quote from the BBC article: Researcher Dr Penny Curtis said: "As adults we make assumptions about what works for children. We found that clowns are universally disliked by children. Some found them quite frightening and unknowable.")