On one hand, it's not a bad thing for renewable power facilities with idle excess to use the excess power on controlled mining, helps pay for themselves and maintenance (if the execs are forward-thinking enough). It also results in a limit as to how far they can expand the mining operation, being as it'd be dependent on spare power being available and average seasonal effects (an issue more for wind and solar, but drought can affect hydroelectric).
On the other, it seems like a waste, considering they could also be putting that same energy reserves into battery storage systems for the rare freak storm, or long-term energy contracts with local charging stations or hydrogen production stations (both would sell too, just not as instantly as mined coin) guaranteeing power (or fuel) even in brownout scenarios.
Maybe a combination of the two could be agreeable? Spare excess electricity to mining to build up some cash reserves for operational and maintenance costs, and also to emergency reserve systems such as battery power plants or dedicated charging facilities.