A word that's not quite as common as its antonym, agelast, someone who doesn't laugh -- though that's a relative commonality, as neither exactly trips off the tongue in everyday life. Or even off the pen, unless one is writing in an erudite persona. The stress falls oddly in this one, to my ear -- most hyper- words put it either on the first syllable of the prefix or of the stem, rather than the -per-, but far be it from me to snigger at this one. I'm not sure whether it was coined from Greek roots or borrowed from Ancient Greek, but it's from hyper-, over + gelastēs, laugher, related to gelaō, I laugh.
We are a country of hypergelasts and frivolists, unable to face any serious issue without mockery and sarcasm.