"A collection of insults 13" joke

An intellect rivaled only by garden tools.

An XT clone in a Pentium zone.

Another engineering prototype that should not have been shipped.

Answers the door when the phone rings.

Any slower and he'd be in reverse. -- Gignac

As a baby his parents stood him on his soft spot.

As bent as a corkscrew.

As bright as a nightlight / small appliance bulb / tulip bulb.

As happy as if he had brains.

As happy as the village idiot.

As much use as a back pocket in a vest. (Very English.)

As much use as a lead parachute.

As quick as a corpse.

As rare as a nine bob note. (Very English.)

As sharp as a marble / bowling ball / beachball / pin head / wet sponge / bowl of Jello / mashed potato sandwich, and twice as smart.