People choose costumes that show how they think of themselves or how they would like to be. Fortunato does not think of himself as a fool but as a clever jester or prankster.
The only serious problem for the protagonist is to lure Fortunato off the crowded streets and down into his catacombs without being recognized. It doesn't matter if Fortunato is recognized by many celebrants, so long as Montresor remains unrecognized as his companion. Poe showed his genius by dressing Fortunato in the most conspicuous costume possible, even providing him with a cap with ringing bells. The costume attracts attention to Fortunato but makes Montresor, in his black cloak and black mask, virtually invisible. He is like Fortunato's shadow. No one will remember seeing him the next day when Fortunato's relatives and friends begin inquiriing about him.
He had on a tight-fitting parti-striped dress and his head was surmounted by the conical cap and bells.
The fact that the costume is tight-fitting and probably has no pockets will make it easier for Montresor to chain him tightly to the rock wall. The costume could contain no concealed weapons or anything the victim might use as a tool to pick the padlock or file at the chains. Here is Montresor's description of the niche and chains:
Within the wall thus exposed by the displacing of the bones, we perceived a still interior recess, in depth about four feet, in width three, in height six or seven.
A moment more and I had fettered him to the granite. In its surface were two iron staples, distant from each other about two feet, horizontally. From one of these depended a short chain, from the other a padlock.
Fortunato will be only four feet from the wall Montresor is building, but his arms would be only about three feet long. If his clothing were a bit looser, or if he had been wearing a cloak, he might have been able to slip out of the tight chains or at least to reach out and push against the wall while the mortar was still wet. No doubt he would wait until Montresor left him alone and might stand some chance of escaping. But Poe has plotted the story in such a way that Fortunato cannot free himself and cannot reach the wall, which is only one foot beyond his reach. It must have been frustrating.
When Fortunato realizes he is trapped, he tries using psychology. He pretends to laugh and says:
"...a very good joke indeed -- an excellent jest."
He represents himself as a connoisseur of jokes and jests. No doubt some injuries suffered by Montresor were laughed off by Fortunato as mere jests. He is a clever scoundrel, but he thinks of himself as an amusing fellow who plays clever jokes. In this story Montresor has turned the tables on his friendly enemy and played an excellent jest on him.
The third paragraph of "The Cask of Amontillado" strongly suggests that Montresor and Fortunato are "gentlemen merchants" dealing in expensive articles such as paintings, antiques, "gemmary," and possibly gourmet wines. Montresor knows Fortunato plans to taste the nonexistent wine, declare it to be ordinary sherry, then rush off to find the source and buy up the entire shipment. He plans to laugh this additional "injury" off as another "excellent jest," as he has often done in the past.
The "thousand injuries" must have been in business dealings. Fortunato is rich, Montresor is poor. Fortunato would always have the advantage. Montresor would have to tolerate him because they would sometimes cooperate in business ventures.