I think it is awfully cruel, and I'm not sure what the purpose is... That being said I can't think of any legitimate legal recourse that could be taken. It isn't illegal, just mean.
Actually, some of these clowns
are being charged for making terroristic threats. The way I see it, it isn't illegal to wear a clown costume, but once you start to intimidate others (esp. with threat of eminent physical violence), you have crossed the line and are now infringing on the other person's rights.
Also, not all of the clown attacks are real. Some police departments have mentioned that sometimes, people will call in a clown attack as a hoax itself. They know with the rise of these cases, police are becoming more vigilant to these attacks causing the call-in hoax to work so well. I'm not sure if these call-in hoaxes are just towards the police or if they are using the police to play a joke on someone else (eg akin to the swatting phenomenon where people would call the police on someone else causing the police / SWAT team to break into the other person's house). Again this is illegal waste of police resources, and some of these call-in hoaxters are being charged for those types of crimes.