The ironically named Fortunato has donned harlequin, the costume of a court fool because it is the Carnival season. Celebrated as such in Italy, this is the same as the celebration of Mardi Gras in other traditionally Roman Catholic areas; that is, it is the period before the beginning of Lent, a six-week period of fasting, abstinence and penance before Easter.
Because of the jubilance and excessiveness of the Carnival, Montresor takes advantage of the fact that he can disguise himself and lure Fortunato into his catacombs with no one paying any attention to their disappearances. Having been celebrating himself, the drunken Fortunato is easily duped into entering the foreboding catacombs because Montesor promises him the opportunity of tasting the Amontillado wine ahead of another connosieur, Luchesi.
Interestingly, the geometric pattern of Fortunato's "parti-striped dress" mimicks the form of Poe's story, the arabesque, a fancifully combined pattern that returns to the initial disturbing idea. For, as Montresor has Fortunato advance deeper and deeper into the catacomb, he alternately cajoles, insults, and cautions Fortunato, all the while leading him farther and farther into the tomb until he fetters him.
"Come," I said, with decision, "we will go back; your health is precious....you will be ill, and I cannot be responsibl. Besides, there is Luchesi-----"
"Proceed," ....herein is the Amontillado. As for Luchesi-"
Indeed, it is appropriate that Fortunato wear the harlequin of a fool, for he is duped into coming to the catacombs, and he is completely tricked into allowing himself to be fettered to a wall.
Poe has Fortunato wearing a costume for several reasons. In the first place, it is the carnival season and everybody on the streets is wearing a costume--with the exception of Montresor himself, who is wearing a black cloak and sometimes a black mask.
Fortunato would not choose a jester's costume to show he thought himself a fool. He chose it because he thought of himself as a jester, a person who enjoyed playing cruel tricks on people. Even when he finds himself in chains in the catacombs, he tells Montresor that it is an excellent jest.
The "tight-fitting" jester's costume makes it obvious that he is unarmed, unlike Montresor who has a rapier concealed under his cloak.
The jester's costume complete with a cap with bells attracts attention, but this has an advantage because it attracts attention away from Fortunato's companion, who is all in black. Lots of people will remember seeing Fortunato when an inquiry is made, but no one will be able to tell who he was with.
The "tight-fitting" costume makes it easy for Montresor to chain Fortunato tightly against the rock wall. The niche is only four feet deep. If Fortunato had any freedom of movement he might be able to slip out from under the chains or to reach out and interfere with the building of the stone wall.
The fact that Fortunato must fancy himself as a jester suggests that at least some of the "thousand injuries" Montresor has suffered were in the form of cruel jests. Montresor is hypersensitive because he is poor and a social outcast, like Poe himself.