fun vs frivolousness

fun and frivolousness both are nouns.

Word	Noun	Adjective	Verb	Adverb
fun	Yes	No	No	No
frivolousness	Yes	No	No	No

As nouns, frivolousness is a hypernym of fun; that is, frivolousness is a word with a broader meaning than fun:

Other hypernyms of fun include frivolity.

fun (noun)	frivolousness (noun)
activities that are enjoyable or amusing	the trait of being frivolous; not serious or sensible
violent and excited activity
a disposition to find (or make) causes for amusement
verbal wit or mockery (often at another's expense but not to be taken seriously)