Fortunato's fool's hat and costume could be interpreted as symbolizing the way Montresor thinks of Fortunato. Montresor believes that he has fooled Fortunato—that he has fooled everyone, really. He claims that he had "given Fortunato [no] cause to doubt [his] good will," though he now plots the man's complete destruction, and his plan is to exploit Fortunato's pride, his "weak point." Montresor will use Fortunato's pride to lure the man into the Montresor family catacombs under the pretense of needing Fortunato's expert opinion on a recent wine purchase. He will make a fool of Fortunato, and he will do it in such a way that Fortunato realizes that he has been made a fool.
With Montresor's skillful manipulation of Fortunato, his own servants, and any potentially prying eyes that might see Fortunato with a man wearing only a dark mask and long, black cape, he seems to think that he is smarter than just about everyone. And maybe he is. He does tell his audience that, according to his own ideas about revenge, he "must not only punish [Fortunato], but punish with impunity." Montresor believes that he has created the perfect crime for which he will incur no negative consequences or punishments. To him, I think, he has made clowns of everyone, and he believes that he is the only truly wise person.
However, we know that Montresor is telling this story some "fifty years" after he killed Fortunato, telling it to someone who "well know[s] the nature of [his] soul"—perhaps a priest. He admits that he "struggled with [the] weight" of the final stone to be fitted before Fortunato's body, even that his "heart grew sick" as he completed his work. Montresor seems to describe and to unknowingly exhibit signs of guilt, meaning that he has not escaped all negative consequences of his crime nor truly achieved successful revenge, based on his own standards. He thought that Fortunato was the clown, but perhaps he has actually made a "fool" of himself as well by believing that a guilty conscience would not catch up to him.