for a "stupid" superhero character, i think of maybe,, endearing in that they fail a lot/cause others to fail on accident, and not specifically a dumb person
for a stupid villain character, i think something similar, eacept they accidentally do good things for people and are really not an evil person, just trying to be a supervillain either because of more serious friends or their situation or somethin,,,, orrrr just over-the-top cartoon villainry pfffft
not really sure bout any specific powers for anythin, so i'll just toss random stuff off the top of my head
- shapeshifting, but possibly limited to limbs n such (say, Sudden Machete Arm:tm:)
- ability to turn gaseous and slip through cracks, alternatively turn into puddle
- chameleon-esque powers of blending into the surroundings
- uuuhhhh a summoning ability for small little creatures and/or minions
- the ability to make clouds appear. tangible and could be stood on, or used as a shield

there's some suggestions for ya!!!