Why is Fortunato's decision to wear a clown hat an interesting and appropriate costume choice by writer Edgar Allan Poe?

Most of the people celebrating the carnival would have been wearing costumes. Poe's narrator Montresor relates of Fortunato:

He had on a tight-fitting parti-striped dress, and his head was surmounted by the conical cap and bells. 

Fortunato has apparently chosen a costume that represents the way he thinks of himself, which is as a witty and funny man. The costume with the cap and bells is ideal for Poe's plot purposes because it is so...

Most of the people celebrating the carnival would have been wearing costumes. Poe's narrator Montresor relates of Fortunato:

He had on a tight-fitting parti-striped dress, and his head was surmounted by the conical cap and bells. 

Fortunato has apparently chosen a costume that represents the way he thinks of himself, which is as a witty and funny man. The costume with the cap and bells is ideal for Poe's plot purposes because it is so conspicuous. If people didn't at first notice the man in the gaudy costume, their attention would be attracted by the ringing bells. No doubt this costume also makes Fortunato seem like a bit of a fool, but for plot purposes it makes him conspicuous and thereby seems to increase Montresor's difficulties in luring him to his palazzo without being noticed as his companion. Montresor has many difficulties to cope with. The story is mainly about how the narrator deals with all the possible things that could go wrong and achieves his ideal revenge.

The fact that the costume is "tight-fitting" is also apparently intended to make it easy for Montresor to chain Fortunato to the granite wall of the catacombs and impossible for Fortunato to escape. If, for example, Fortunato were wearing a suit and cloak like Montresor, he would have some chance of slipping out of the chain around his waist; or he might have more freedom to reach out and push against the stone wall, either while Montresor was erecting it or after Montresor had left and the mortar was still damp. Also, the fact that the costume is "tight-fitting" suggests that Fortunato has nothing with which he might be able to file at the chains or pick at the padlock. Such a costume would probably not even have any pockets.

Montresor disposes of the question of his motive for killing Fortunato very quickly. Poe evidently wants to make it clear that this is a perfect-crime story. The biggest problem is to lure Fortunato to Montresor's palazzo without being noticed as his companion, and to keep Fortunato drunk and distracted until he is chained to the rock wall. The gaudy costume with its cap and bells would seem to present a problem. This actually serves as an advantage, however, because it attracts all the attention to Fortunato and allows Montresor to go unnoticed. Montresor is wearing a dark cloak and a black mask. He would look almost like Fortunato's shadow as the two men passed through the crowds of intoxicated celebrants. Many people would remember seeing Fortunato when the inevitable inquiry began, but nobody would remember seeing anyone with him.

Because of the tight-fitting costume, Montresor can see at a glance that Fortunato does not have any weapon, visible or concealed. Once Montresor gets Fortunato down into his wine vault, he has him at his mercy. Montresor has a rapier concealed under his roquelaire. He can kill his victim at any time, although it would be more convenient to lead him to the crypt where everything is waiting for his entombment rather than to drag his dead body through a series of dark tunnels.