part 5 had a lot of ugly-ass designs. Like Fugo. suit full of holes, why the fuck. but Diavolo takes the cake for me. looks NOTHING like a mafia boss at all, and a big let-down when he gets revealed.
You're absolutely right, disappointment is a big part of it. After all, 'the boss' spends the majority of the story in the shadows raising our expectations… but then we actually see Diavolo and he looks like a male stripper clown with spots of what I can only assume is mold in his hair. None of his clothes match up at all, while Fugo, Melone and Rykiel at least have consistency in their design. It felt like a pretty desperate attempt to make a bland character interesting.