Yes, the Democratic primary in California was rigged for Hillary Clinton. Millions of votes cast by supporters of Bernie Sanders in California were simply not counted. The DNC, which includes several party leaders in California, has been working from the beginning to prop up Hillary Clinton and to place the nomination in her lap, and they've committed election fraud there.
Conspiracy theories abound in regards to whether certain electoral processes are "rigged" in favor of one side or the other. In the case of the California Primary, these theories are justified. All the "propositions" that came along with the primary vote were specifically engineered to confuse the average voter and therefore bring victory to the Democrats.
The California Primary was rigged because the common citizen doesn't actually have control over what is going on. Due to the outdated Electoral College, citizens don't receive the voice that the original Founders envisioned for their people. The entire system is rigged, and California is just one primary example of that happening.
I would like to think that the California Primary was not rigged, although we will never know. Politics as a whole is very corrupt, and personally I would not believe anything that a politician has to say, and would never take what they say at face value. However, I would like to think that it is not rigged, even though it probably is.