This is definitely not a conclusive list...
1. A few clowns short of a circus.
2. A few fries short of a happy meal.
3. The wheel's spinning, but the hamster's dead.
4. All foam, no beer.
5. The butter has slipped off his pancake.
6. The cheese slid off his cracker.
7. Body by Sony, brains by Mattel.
8. Warning: Objects in mirror are dumber than they appear.
9. Couldn't pour water out of a boot with instructions on the heel.
10. Fell out of the stupid tree and hit every branch on the way down.
11. As smart as bait.
12. Doesn't have all his dogs on one leash.
13. Her sewing machine's out of thread.
14. One fruit loop shy of a full bowl.
15. Her antenna doesn't pick up all the channels.
16. His belt doesn't go through all the loops.
17. Proof that evolution CAN go in reverse.
18. Receiver is off the hook.
19. Not wired to code.
20. Skylight leaks a little.
21. Her slinky's kinked.
22. Too much yardage between the goal posts.
23. Got a full 6-pack, but lacks the plastic thingy to hold them together.
24. A photographic memory, but the lens cover is on.
25. During evolution his ancestors were in the control group.
26. Gates are down, the lights are flashing, but the train isn't coming.
27. Is so dense, light bends around her.
28. If brains were taxed, he'd get a rebate.
29. Standing close to her, you can hear the ocean.
30. Some drink from the fountain of knowledge, but he just gargled.
31. One sandwich short of a picnic.
32. Not the sharpest knife in the drawer.
33. One of her DuraCells is in upside down.
34. His elevator doesn't stop at all the floors.
35. One taco shy of a combo platter.
36. One beer short of a six pack
37. The porchlight's on but no one's home