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.