No. There is a common bias that a motorcyclist must have been speeding or riding recklessly, and insurers lean on it — but a bias is not evidence. Fault is decided by what each person actually did, and in many rider crashes the other driver failed to yield, merged without looking, or turned across the rider's path.
California follows a pure comparative-fault rule, which means that even if you are found partly responsible, you can still recover, with your compensation reduced by your share of the blame. So being assigned some fault does not end your claim — it affects the amount.
The way to answer the 'reckless rider' assumption is with proof: the crash report, witness accounts, vehicle damage, and where needed, accident reconstruction. Building that record is a core part of representing an injured rider.