These sound better than saying that "<span style="font-weight: bold">someone is stupid</span>."
1. A few clowns short of a circus
2. A few fries short of a Happy Meal
3. An experiment in artificial stupidity
4. A few beers short of a six-pack
5. Dumber than a box of hair
6. A few peas short of a casserole
7. Doesn't have all his cornflakes in one box
8. The wheel's spinning but the hamster's dead
9. One Fruit Loop shy of a full bowl
10. One taco short of a combo plate
11. A few feathers short of a whole duck
12. All foam, no beer
13. The cheese slid off the cracker
14. Body by Fisher - Brains by Mattel
15. Room temperature IQ
16. Warning: Objects in mirror are dumber than they appear
17. Couldn't pour water out of a boot with instructions on the heel
18. He fell out of the stupid tree and hit every branch on the way down
19. An intellect rivaled only by garden tools
20. As smart as bait
21. Chimney's clogged
22. Doesn't have all his dogs on one leash
23. Doesn't know much but leads the league in nostril hair
24. Elevator doesn't go all the way to the top
25. Forgot to pay his brain bill
26. Her sewing machine's out of thread
27. His antenna doesn't pick up all the channels
28. His belt doesn't go through all the loops
29. If he had another brain it would be lonely
30. Missing a few buttons on his remote control
31. No grain in the silo
32. Proof that evolution CAN go in reverse
33. Receiver is off the hook
34. Several nuts short of a full pouch
35. Skylight leaks a little
36. Slinky's kinked
37. Surfing in Nebraska
38. Too much yardage between the goal posts
39. He's not the brightest light in the harbor
40. He's not the brightest crayon in the box
1. A few clowns short of a circus
2. A few fries short of a Happy Meal
3. An experiment in artificial stupidity
4. A few beers short of a six-pack
5. Dumber than a box of hair
6. A few peas short of a casserole
7. Doesn't have all his cornflakes in one box
8. The wheel's spinning but the hamster's dead
9. One Fruit Loop shy of a full bowl
10. One taco short of a combo plate
11. A few feathers short of a whole duck
12. All foam, no beer
13. The cheese slid off the cracker
14. Body by Fisher - Brains by Mattel
15. Room temperature IQ
16. Warning: Objects in mirror are dumber than they appear
17. Couldn't pour water out of a boot with instructions on the heel
18. He fell out of the stupid tree and hit every branch on the way down
19. An intellect rivaled only by garden tools
20. As smart as bait
21. Chimney's clogged
22. Doesn't have all his dogs on one leash
23. Doesn't know much but leads the league in nostril hair
24. Elevator doesn't go all the way to the top
25. Forgot to pay his brain bill
26. Her sewing machine's out of thread
27. His antenna doesn't pick up all the channels
28. His belt doesn't go through all the loops
29. If he had another brain it would be lonely
30. Missing a few buttons on his remote control
31. No grain in the silo
32. Proof that evolution CAN go in reverse
33. Receiver is off the hook
34. Several nuts short of a full pouch
35. Skylight leaks a little
36. Slinky's kinked
37. Surfing in Nebraska
38. Too much yardage between the goal posts
39. He's not the brightest light in the harbor
40. He's not the brightest crayon in the box
