Both Uses of

mischievous
in
Northanger Abbey

But Mrs. Morland knew so little of lords and baronets, that she entertained no notion of their general mischievousness, and was wholly unsuspicious of danger to her daughter from their machinations.†
Chpt 2

mischievousness = the quality of playfully causing minor trouble

You really have done your hair in a more heavenly style than ever; you mischievous creature, do you want to attract everybody?†
Chpt 10 *

Definitions:

(1) (mischievous)
playfully causing minor trouble; or describing the smile of someone doing so

Much less commonly, mischievous can reference real harm without any sense of fun. But in modern writing, that usage has largely shifted to other words like malicious, destructive, or damaging.
(2) (meaning too rare to warrant focus)
meaning too rare to warrant focus:

In law, mischievous references a property crime such as vandalism or graffiti. Very rarely, the word can reference someone or something causing serious damage.

In archaic literature mischievous often refers to bad behavior without any connotation of playfulness or of the harm being minor.