Besides the absurdity of mining coal everywhere in game, why would it not be valid? In regions, where coal layers are accessible from the ground, often an illegal shafts are being dug out by individuals to get free coal. Rock coal is relatively soft, so you don’t need sophisticated equipment to mine it on a small scale, its layers are consistent, so you don’t sift through dug-out rock or use some special process like in gold mining etc., where you would need special machinery etc. And its relatively caloric as fuel, compared to wood, etc. In my opinion in a region where you could have access to coal, it would be valid even for a sole survivor. Of course its a game of opportunities and economics of time - if in a same time you could just get some wood, gas, etc. then you lose incentive to dig for it. But it’s still an option.
