Longtime mountain resident here. Really hoping to see lots of Supercharger stalls in Big Bear, given the number of Tesla visitors we see! But this will take time, given the site and infrastructure requirements.
For what it’s worth, I regularly use FSD Beta on mountain roads, mainly when traffic is slow or when there’s no one behind us. Late at night, I’ve let FSD drive us up CA-330 all the way from Highland to Running Springs. It slows excessively for curves, but it does work! It has improved considerably over the last year.