The dam (GERD) is only partially completed. They started filling, and last yera began generating electricity, but for now they have only two turbines, (producing ~700 MW) operating. The "nameplate" capacity is 6469MW but anything near that capacity will only occur seasonally, and even a high sustained level would currently be "politically & financially awkward to use."

There are transmission lines from Kenya to purchase (take) some of GERD's generation, but I'm not sure if those reach the dam yet, and since Kenya already has excess generation capacity, it's unknown at what point Kenya might be a customer.

But yes, at some point it makes absolute sense for the dam to generate revenue from the electricity generate from GERD that exeeds what is in demand by the grid by selling that excess to bitcoin miners.

But currently, Ethiopia has significant internal conflict. Until that is resolved, this issue will likely dissuade investors from building a mining op even if there was electricity available today.