All 3 Uses of

mischievous
in
Utopia, by Thomas More

Now what if, after all these propositions were made, I should rise up and assert that such counsels were both unbecoming a king and mischievous to him; and that not only his honour, but his safety, consisted more in his people's wealth than in his own; if I should show that they choose a king for their own sake, and not for his; that, by his care and endeavours, they may be both easy and safe; and that, therefore, a prince ought to take more care of his people's happiness than of his own, as a shepherd is to take more care of his flock than of himself?†

All men are not so wise as to choose a woman only for her good qualities, and even wise men consider the body as that which adds not a little to the mind, and it is certain there may be some such deformity covered with clothes as may totally alienate a man from his wife, when it is too late to part with her; if such a thing is discovered after marriage a man has no remedy but patience; they, therefore, think it is reasonable that there should be good provision made against such mischievous frauds.†

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.