I think CO2 has been commandeered as the go to metric for environmental impact because it occurs in a wide range of scaled processes. But it does paint a really incomplete picture of the situation. Perhaps a more holistic indicator could be something like environmental entropy production. Some metric that describes the rate of environmental disordering, which stresses the regulatory mechanisms that exist in our environmental ecosystem (e.g. CO2 absorption and transformation or pollinator populations that sustain flora populations).

So lets just use "impact" as a general term for human activity disordering the environmental state. Whilst the Bitcoin consumes electricity, it provides a valuable service. That is the security consensus of the PoW method which is enforced by thermodynamics. Some form of environmental impact in the form of electricity consumption, hardware manufacturing, etc. is of course acceptable. The amount of impact should be equal to the intrinsic value of processes that Bitcoin is used for (currently speculative, but the implementation horizon is fast approaching).