What is a word that means "lacking seriousness"? [closed]

"Frivolous" maybe?

I think it depends on what you consider as seriousness, like "depth" or serious mood.

Google translate also yields

flippant, glib, facetious, joking, jokey, lighthearted, fatuous, inane, senseless, thoughtless, flip; time-wasting, pointless, trivial, trifling, minor, petty, insignificant, unimportant, de minimis

flippant - (adj) "lacking proper respect or seriousness" - MW, frivolously disrespectful or lacking in seriousness. TFD

• "He made a flippant response to a serious question."

• "His flippant comment offended many people."

• "You have many talents but you are a flippant person."

is silly seems simple. The thesaurus yields wonders from there.