Even if it is clean energy (and assuming this is mining without any valuable processing like verifying transactions):

1. Waste is waste and there's only a finite amount of clean energy that can be used. The sun only outputs so much usable energy per day and daming rivers does not happen without consequence.

2. This generates alot of heat. People aren't mining in their house so this is concentrated areas of heat pollution. It may be negligible but environmental hotspots wreak havoc on local ecosystems, especially when dispersed in aquatic ecosystems.

When electricity is generated by burning fossil fuel or garbage, waste heat is fed to centralized heating systems. In this way efficiency of over 90% is achieved. Probably it's time for the next stage of this idea: electricity should be immediately fed to mining facility, which preheats water that goes to the furnace. In the end all computers are just hi-tech heaters.

Also, there is always a hidden opportunity cost. Bitcoin using green energy would still increase worldwide energy consumption, and simply push other energy consumers onto more dirty sources.