Poe dresses Fortunato as a fool because he followed Montressor into the catacombs in hopes of sampling a rare bottle of wine which are the actions of a fool. In the end he allows Montressor to manipulate him and kill him without realizing the real intentions of his "friend" until it is done.
In the short story "The Cask of Amontillado" by Edgar Allan Poe, the author describes Fortunato's clothes as a traditional jester costume, "The man wore motley. He had on a tight-fitting parti-striped dress and his head was surmounted by the conical cap and bells". The author describes Fortunato's jester costume because it symbolizes his foolishness.
