So, overall, a tiny minority; sure, it's a somewhat less tiny minority in some counties (and a significant one in a few) but those are mostly very low population counties, too.
State-wide that's true. But herd immunity is a local issue. So if your kid is going to a school where more than 10% of the other kids are unvaccinated then they won't benefit from herd immunity, despite the fact that the number of those unvaccinated kids is a tiny minority of all California kids.