Antonyms for earnest

serious; very important:

inconstant, unfixed, wobbly, untrustworthy, weak, loose, wavering, unstable, unsteady, irresolute, changeable.

very enthusiastic:

cool, unserious, unenthusiastic, insincere, frigid, unconcerned, unpurposeful, cold, unexcited, indifferent, trivial, flippant, apathetic, thoughtless, dispassionate, unimportant, Unpassionate, unthoughtful.

noun:

cockeyed, daft, phlegmatic, wacky, careless, mad, negligent, nonsensical, uninterested, stony, balmy, half-witted, unmindful, insane, brainless, kooky, indifferent, jerky, asinine, joking, Weak-minded, cold, cuckoo, unmoved, lunatic, crazy, uproarious, regardless, jesting, zany, unwise, fatuous, purposeless, senseless, loony, apathetic, nutty, foolish, cool, dotty, witless, stolid, sappy, calm, heedless, dispassionate, kittenish, screwball, Daffy, frigid, hilarious, light.