Not necessarily. The driver who caused the crash can still be responsible for your injuries whether or not you were wearing a helmet. California requires helmets only for certain riders, and being without one is not the same as being at fault for the collision itself.
Where a helmet was not worn, an insurer may argue it made a head injury worse and try to reduce that part of the claim. Whether — and how much — that applies depends on your injuries and the law, and it does not erase a claim for a crash someone else caused.
So the helmet question is usually about damages, not about who caused the crash. Separating the two, and answering the argument with the facts, is part of protecting a cyclist's claim.