Both Uses of

mischievous
in
The Mill on the Floss

He knew all about worms, and fish, and those things; and what birds were mischievous, and how padlocks opened, and which way the handles of the gates were to be lifted.†
Chpt 1.5 *

He's never behaved quite so pretty to our family as he should do, and the children take after him,—the boy's very mischievous, and runs away from his aunts and uncles, and the gell's rude and brown.†
Chpt 1.9

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.