ridiculous

asinine, brainless, dumb, fatuous, foolish, half-baked, half-witted, harebrained, idiotic (also idiotical), imbecile (or imbecilic), inane, jerky, moronic, nonsensical, simpleminded, stupid, unwise, weak-minded, witless

balmy, cockeyed, crazy, cuckoo, daffy, daft, dotty, insane, kooky (also kookie), loony (also looney), lunatic, mad, nutty, screwball, senseless, wacky (also whacky)

fantastic (also fantastical), far-fetched, inconceivable, incredible, unbelievable, unreal, unrealistic, unreasonable

illogical, irrational

amusing, diverting, entertaining

clownish, knockabout, slapstick, slapsticky, zany

facetious, flip, flippant, pert, smart, smart-aleck, smart-alecky, snickery

jocular, playful, waggish

campy, jokey (also joky)

priceless, rich, whimsical, witty, wry

blithesome, gleeful, jocose, jocund, jolly, jovial, laughing, mad, merry, mirthful