In "The Cask of Amontillado", Edgar Allan Poe chooses to dress Fortunato's character as a clown, a jester, during the carnival celebrations. Fortunato is wearing this costume while he is out drinking and enjoying the carnival. The author choose the clown costume to use it as a symbolism of the character's foolishness, one that can be associated with clowns.
At the same time, the fact that Fortunato is wearing a clown costume while acting like one, helps to reinforce the concept of why Montressor is going to take advantage of Fortunato's character.