These are actual phrases from Officer Efficiency Reports (performance appraisal for the military officers).
"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 plastic 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 outscore him in IQ tests."
"Donated his brain to science before he was done using it."
"Fell out of the family tree."
"Gates are down, the lights are flashing, but the train isn't coming."
"Has two brains: one is lost and the other is out looking for it."
"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."
"If you stand close enough to him, you can hear the ocean."
"Some drink from the fountain of knowledge; he only gargled."
"Takes him an hour and a half to watch 60 minutes."
"Was left on the Tilt-A-Whirl a bit too long as a baby."
"Wheel is turning, but the hamster is dead."