I read on Wikipedia some funny things about Cleveland boss James Licavoli. If they are true, he is without question the dumbest Mafioso ever.
It says he was once caught switching the price tags on a pair of pants and another time for using slugs on machines. He also used stolen credit cards on vacations. All of this while being the boss of a crime family and in spite of being very rich.

Willie Marfeo to Henry Tameleo:

1) "You people want a loaf of bread and you throw the crumbs back. Well, fuck you. I ain't closing down."

2) "Get out of here, old man. Go tell Raymond to go shit in his hat. We're not giving you anything."