Well, that's actually a bit of a trick question depending on which version you're talking about! In Victor Hugo's original novel, Clopin isn't really a major character by that name. The name 'Clopin Trouillefou' appears, but he's a minor character, the King of the Court of Miracles. The vibrant, jester-like Clopin everyone pictures? That's almost entirely a Disney creation.
In the 1996 animated film, Clopin is the gypsy puppeteer and unofficial master of ceremonies for the story. He narrates the tale, serves as a sort of Greek chorus, and leads the 'Court of Miracles' song. He's this fantastic, morally ambiguous figure—entertaining and charming, but also fiercely protective of his people and not above a bit of menace. I think what makes him so memorable is that he bridges the gap between the audience and the dark, complicated world of the story. He's our guide, but he's also deeply embedded in the conflict, which is a clever narrative choice. His final line, 'So here's a riddle, what makes a monster and what makes a man?' really drives home the film's central theme long after the credits roll.
Honestly, I prefer the Disney version because he adds a layer of theatrical flair and sardonic commentary that Hugo's darker, grimmer novel obviously didn't need. He turns the story into a performance, which fits the cathedral setting perfectly.