I just want to provide a slightly alternative answer, because I'm not sure if this is all that black and white like other answers would paint it.
First of all, if the mining (or indeed whatever else!) is done at home, outside of work hours, while not connected to any company resources, no company resource is being spent and no damage is done to the company. Not even close.
Secondly, unless the company is in the crypto business, there's no conflict of interest, so that wouldn't be an issue either.
Thirdly, you aren't spending your brain resources on it, or losing sleep over it (hopefully!) so it wouldn't affect your job performance. And even if it did, it will affect you more than them, so you'd have to think for yourself.
Of course, like others suggested, if you aren't sure, go ahead and ask your manager (in writing!), but chances are they wouldn't know what it is and by default their answer will be a 'no'. But to me it sounds like going and asking your manager (in writing!) if you could play Minecraft on your Macbook while at home. Or if you could watch Youtube. Absolute nonsense.
PS: Will those down-voting provide any commentary please? I'm curious as to the reasons they have in their heads.
To start off I'll give you my own reasons: if I give someone a Mac to do some work, all I care about is this work being done to the agreed quality and timelines. What they do in their spare time is none of my business and I know for a fact that this Mac will be a write-off and essentially garbage within 24 months anyway. Even if it'd taken some superficial damage to the fans from overspinning (🙄), like some people are theorizing here.