From the Super Mario Wiki
A Smiley Bomb is an explosive item from Wario World. It is a generic, purple-colored bomb with, as its name suggests, a smiley-face painted on it. Being a bomb, it has a fuse and will explode after a while, even if Wario does not touch it. The Smiley Bomb is found in Unithorn's Lair commonly, as it is used as a dud.