Clown, the alternate identity for Violator, is a demon who's main task is to direct Hellspawn to fulfill Satan's desire. Holding a deep hatred towards Spawn for being given the position of General by Malebolgia to lead his army of hellish nightmares, his goal is to screw around with Spawn so much to give into his hellish powers for pure evil, trying to steer him away from using them for vengeance.
Michael Nicolosi sounds absolutely frightening and has this sort of sneery tone/delivery mixed with a "creepy clown" voice. You just get lost in the way he chews the scene up and spits it out at Spawn. I really wish he took more voice roles, but Clown is one definite win for him.
The other two Mario Bros. actors are give or take. Charles Martinet is hindered by the fact he's a first level boss as well as the Game Boy Color's awful sound quality, but I think he got the nature of Clown being this absolute scumbag well, if not sounding like Waluigi and Wario.
John Stocker does this Krusty the Clown voice and it fits Clown really well, but I just get Toad memories whenever I hear him laugh. Nothing personal against him, he could probably do Clown better with more material than Charles.