These are actual lines from military performance appraisals or Officer Efficiency Reports (OERs).

•Not the sharpest knife in the drawer.
•Got into the gene pool while the lifeguard wasn't watching.
•A room temperature IQ.
•Got a full 6-pack, but lacks the thingy to hold it all together.
•A gross ignoramus -- 144 times worse than an ordinary ignoramus.
•A photographic memory but with the lens cover glued on.
•A prime candidate for natural deselection.
•Bright as Alaska in December.
•One-celled organisms out score him in IQ tests.
•If you stand close enough to him, you can hear the ocean.
•Fell out of the family tree.
•Gates are down, the lights are flashing, but the train isn't coming.
•He's so dense, light bends around him.
•If brains were taxed, he'd get a rebate.
•If he were any more stupid, he'd have to be watered twice a week.
•If you give him a penny for his thoughts, you'd get change.