Actually, this phobia is quite widespread. In most cases this phobia refers to children but it is surprisingly common for adults as well. That is why the image of "scary clown" is often used in horrors. The famous example is the film adaptation of Stephen King's "IT".

Did you know that Johnny Depp is one of the famous phobia's sufferers? He said once that it is impossible to distinguish if the clowns are really kind/funny or if they are about to bite your face off.