If governments forced miners to use solar energy only, would Bitcoin become more sustainable and valuable, or would it reduce mining capacity drastically?
I don't see the need for the government to impose such rules to miners. It would be at their own loss since they make money as well from the electricity consumed by those mining farms. I would reply directly to the title of this topic, solar energy has long been a back up option for miners.
Mining consumes a lot of energy and since it is a process that has to run 24/7, they have to consider variety of energy sources. In some countries, they barely have enough solar energy to power their solar farms. So it won't make much sense to move completely to solar energy. One has to put other things into consideration...
If they have such mandate, I would say miners would try to comply such directive. And more than likely, small time miners may stop their operations as they can't afford such route. As it needs a good amount of money to set-up the solar system, they may try to find ways how to go around on this requirement. But in today's generation, solar set-up is getting cheaper, hence, the possibility of utilizing it is already doable.
The truth is this route is indeed a more sustainable path, hence, this directive is actually a very promising one if miners can go to this path 100%. However, it would take time for all miners to adopt such route.