No idea how many are doing so, but there are plenty of skiers here in the Bay Area and Sacramento areas, and it stands to reason that people who do so are more likely to care about air pollution than the general public. I've seen my share of BEVs in Yosemite, including a Model X in the same overnight wilderness trailhead parking area (at a downhill resort) I left my car in on my most recent trip.
FCEV Claritys are sold in Northern California as well as Southern California, and you can certainly drive them to Tahoe to ski, as there are stations in Sacramento as well as Truckee. Getting from the Bay Area to Yosemite and back remains very difficult for an FCEV - a Nexo and probably a Clarity could do it with a lot of care, a Mirai no. As I usually go towards Yosemite and almost never to Tahoe anymore, it's not surprising that I haven't seen any Claritys in the mountains. As for SoCal, there are ski areas within round-trip FCEV range of L.A.