Best antonyms for 'serious' are 'funny', 'unimportant' and 'minor'.

Synonyms for SERIOUS: solemn, grave, sober, earnest, sedate, heavy, staid, alarming, austere, businesslike; Antonyms for SERIOUS: cheerful, happy, funny, light, lighthearted, flippant, unimportant, trivial, minor, frivolous.

In the sense of solemn or thoughtful in character or manner he had a serious expression on his face Synonyms solemn • earnest • grave • sober • sombre • unsmiling • poker-faced • stern • grim • dour • humourless • stony-faced • thoughtful • preoccupied • deep in thought • pensive • meditative • ruminative • contemplative • introspective • staid • sedate • studious • bookish Opposites light-hearted • cheerful • jovial

In general, adjectives and adverbs have opposite meanings, that is, words reporting quality and quantity often have opposite words. Serious means; grave, critical, important, earnest, solemn, major, considerable, heavy, sober