fun vs mischievousness

fun and mischievousness both are nouns.

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

As nouns, mischievousness is a hyponym of fun; that is, mischievousness is a word with a more specific, narrower meaning than fun:

fun (noun)	mischievousness (noun)
activities that are enjoyable or amusing	reckless or malicious behavior that causes discomfort or annoyance in others
violent and excited activity	an attribute of mischievous children
a disposition to find (or make) causes for amusement	the trait of behaving like an imp
verbal wit or mockery (often at another's expense but not to be taken seriously)