'Stupid' is not a nice word. Unkind if you will. Here's a few suggestions if you need to get your point across but don't want to be mean... Well, that mean.
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 combination plate.
11. A few feathers short of a whole duck.
12. All foam, no beer.
13. The cheese slid off his cracker.
14. Body by Fisher, brains by Mattel.
15. Has an IQ of 2, but it takes 3 to grunt.
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 floor.
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.