If the idle game is running at 100% there is no power left to also crypto mine. If the idle game runs at 10% you could use 90% for mining but then you would pay for 100% energy usage instead of 10%. You'd probably end up paying more than you'd mine.
Yeah I was mostly thinking of it as some clever way to mine. Somehow linking the mining engine into the gameplay itself. That's a proper point though when just looking at the efficiency side of things.