Well sure, if the electricity is powering something you already need and the waste heat is beneficial, then awesome, I guess that's free heat. But it's actually pretty rare that people need to be using that much electricity for anything as consistently as you would need for heating a home. And if you don't actually need to be using that electricity, there's really no way around the fact that electric heating is really pretty expensive.
I guess if you are stuck with electric heating as your only option and a heat pump is out of your price range, then mining crypto could be a nice way to offset the cost of electric heating... But then the equipment costs would add up and you'd probably be better off with a heat pump anyway.