I would not expect miners to voluntarily stop mining just because the user agent reports low battery. Any miners that care about mobile users could check the user agent string and throttle mining.
If the battery API ever becomes a thing, the first thing I will do is to hack the browser to perpetually report an almost-dying battery. Similarly, if there ever comes a standard for network link quality, I'll make my browser report I'm on a 56k modem.
The reason is simple: whatever cruft the site decides to only serve to high-end users is the cruft I'm very much not interested in seeing.
And because I'm definitely not alone in this thinking, those APIs will be failures for anything other than getting additional bits for tracking and profiling people.
It'd be fine. You would do that, I would do that and most people here would do that. The vast majority of users would not, it's just like adblockers: only a minority uses them.
Meh, then you're back to the same dilemma ads have: you have to convince the users to visit your page from a desktop PC as much as possible. And that usually means making the mobile experience as bad as possible. Which the users won't like for fairly obvious reasons.
Pretty sure user anger didn't stop the prevalence of pop-up Punch The Monkey adverts. The massive rise in adblocking forced the ad industry to stop being quite so obnoxious.
So, yeah, relying on advertisers to not piss off users is a bit much given their business model relies on interrupting your attention.