I'm with stupid

Moronic, foolish, senseless, brainless, mindless, idiotic, imbecile, insane, lunatic, asinine, ridiculous, ludicrous, absurd, preposterous, silly, inane, witless, half-baked, empty-headed, unintelligent, slow-witted, weak-minded; informal crazy, dumb, brain-dead, cretinous, imbecilic, doltish, thick, thickheaded, birdbrained, pea-brained, pinheaded, dopey, dim, dimwitted, halfwitted, dippy, fat-headed, blockheaded, boneheaded, lamebrained, chuckleheaded, dunderheaded, muttonheaded; daft, dumb-ass, chowderheaded, ignorant, dense, brainless, slow, dull, simple, stunned, vacuous, vapid, bovine, dozy, fat-headed, birdbrained, simpleminded, idiotic, obtuse, doltish, cretinous, soft in the head, boneheaded, wooden-headed, scatterbrained, nonsensical, unthinking, ill-advised, ill-considered, unwise, injudicious, laughable, risible, fatuous, mad, cracked, cockeyed, harebrained, nutty, batty, cuckoo, loony, loopy.

If you want to impugn someone's intelligence, the options are almost limitless. Thank god for that!