I'm a sucker for seemingly bad/not very useful superpowers!
The silliest I've ever made was a kid who will explode whenever he sneezes (he survives, but his surroundings usually don't). Doesn't help that I gave him pollen allergies…
I have a character who can fly, which probably doesn't sound too silly on its own. What makes it silly is that the character in question is very afraid of heights, and also very insecure about his power. So, he's never actually flown despite being physically able to, which basically just reduces his power to being able to grow a pair of wings on command.
One of my flying characters can only fly when she's listening to music. Another character is a teleporter with A.D.H.D so it sometimes makes him teleport at random.