unsmiling

antic, comic, comical, droll, farcical, funny, hilarious, hysterical (also hysteric), laughable, light, light-headed, ludicrous, ridiculous, riotous, risible, screaming, sidesplitting, uproarious

featherbrained, flighty, frivolous, goofy, harebrained, lighthearted, puerile, scatterbrained

absurd, asinine, balmy, brainless, cockeyed, crazy, cuckoo, daffy, daft, dotty, fatuous, foolish, half-witted, insane, jerky, kooky (also kookie), loony (also looney), lunatic, mad, nonsensical, nutty, preposterous, sappy, screwball, senseless, silly, unwise, wacky (also whacky), weak-minded, witless, zany