Just that. Clowns hate tangelos. Nothing else on the billboard. Just "Clowns hate tangelos".
Googling tells me that it's from Grim Adventures of Billy and Mandy... and that an identical billboard was sighted near Chicago.
So I'm posting this not just to go "erf?", but to create another link for Google to pick up, so we can eventually solve the mystery. Clowns hate tangelos. Why is this on a billboard?