What does Fortuanto's costume symbolize?

A court jester may have been considered a fool, a man with an abnormally low IQ, but Fortunato would not have chosen to wear a jester's costume if he wanted people to take him for a moron. A court jester was privileged to make jests, to play jokes on people, to make other people look foolish. In masquerades people choose costumes to represent what they would like to be or what they think they are. Fortunato thinks of himself as a jester, a man who loves to make jests, even though some of his jests may be painful. The thousand injuries Montresor has suffered may have come from what Fortunato considered "jests." Montresor was vulnerable because he was an outsider, a Frenchman, and because he was living on the verge of poverty. What Fortunato considered a jest, such as commenting on the threadbare coat Montresor was wearing, might have been extremely painful for Montresor himself--and he might have realized that Fortunato knew it was painful.

When Fortunato finds himself chained to the wall, he pretends to believe it is a joke. He says:

"Ha! ha! ha! -- he! he! -- a very good joke indeed -- an excellent jest."

He should know that Montresor is not the jesting type. He is giving Montresor an excuse to change his mind and release him. He is desperate and using every ounce of his intellect to figure a way out. He says:

"But is it not getting late? Will not they be awaiting us at the palazzo, the Lady Fortunato and the rest?"

He wants to plant the idea that people will be missing him and perhaps sending out searchers. He also wants to plant the idea that a lot of people know he and Montresor were together. He realizes that Montresor wants to leave a cold trail and would like Fortunato to be missing at least overnight, if not for several days.

Poe certainly doesn't want the reader to think Fortunato is a fool. That would make his crime look too easy. Montresor says early on that Fortunato "...was a man to be respected and even feared." Montresor would have plenty of reason to fear him if he ever got out of those chains!