About 39 million people live in California that is total people, including kids, immigrants, people not registered to vote, etc. Even if you assume that all those people could vote, and that they all voted the same way... well it still isn't enough to elect a president. In this election which had a pretty mediocre turnout, each major party candidate got more than 60 million votes (by way of comparison President Obama got almost 70 million in 2008).
But like I said, not all can or do vote. Of those 39 million people, only somewhere in the realm of 12-16 million actually do vote, numbers vary by year but are in line with the overall trend in the US of pretty low voter participation. So when you look at it California accounts for about 12% of the population in the US and what do you know, about the same percentage of the popular vote, which would seem to be the literal definition of fairness in terms of "one person, one vote".
However, even that large voting block doesn't matter since it turns out California is not unified, no state is. Maybe you get the mistaken impression it is because of the EC results, but that is only because of the "winner take all" nature of elector allocation. California likes democrats as a whole, but not universally. This year 7.2 million people there voted Clinton, 3.8 million voted Trump.
So no, it wouldn't decide an election in a popular vote, not even close.