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...