Fortunato shows that he is a heavy drinker as well as a exhibitionist. He enjoys the carnival season because it gives him the freedom to be an even heavier drinker and more of a show-off than usual. The fact that he is wearing a jester's costume does not mean that he wants to be taken for a fool. Rather, it means that he considers himself a funny man, a jester. When people choose costumes for such events, they generally choose one that represents what they would like to be (e.g. Superman). We can imagine that Fortunato is disporting himself by throwing confetti in people's faces, blowing a tin horn in people's ears, and perhaps squirting them with water. He likes practical jokes. When he finds himself chained to the granite wall his first thought is to say:
“Ha! ha! ha!—he! he! he!—a very good joke, indeed—an excellent jest. We will have many a rich laugh about it at the palazzo—he! he! he!—over our wine—he! he! he!”
He does not really believe this is a joke, but because of his character and reputation of a practical joker he can pretend to construe it as a jest in order to give Montresor an excuse to release him if Montresor is having second thoughts about going through with this murder. The thousand injuries which Montresor speaks of at the beginning of the tale were probably cases where Fortunato cheated Montresor in business deals and then laughed it off as "an excellent jest." He is not a funny man but a cruel man who pretends to be good humored and comical. He is planning to cheat Montresor on this Amontillado tasting. If it is genuine he will say it is only ordinary sherry. This will discourage Montresor from buying any more at a "bargain" price, and Fortunato can easily find the newly arrived Spanish ship and buy up the whole cargo of Amontillado himself. When Montresor finds out, Fortunato will call it "an excellent jest." But this time the joke will be on Fortunato, because there is no Spanish ship and no Amontillado.