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.