I'm not a lawyer, but I'm pretty sure making a clone of Super Mario Bros is illegal. Copying any Nintendo game is definitely playing with fire...especially if you plan on distributing it. I've seen Nintendo aggressively protect their copyrights in the past. Are you really willing to risk that?
Mind you, you could get away with it if you switch out the graphics. As far as I know, the general game mechanics can't be copyrighted.