Does a California eviction case affect my credit score?

Not directly. There's no mechanism that reports an eviction filing itself to the credit bureaus the way a missed payment gets reported.

The real exposure runs through two side doors instead: a money judgment for unpaid rent can end up with a collections agency and reported that way, and separately, tenant screening companies, the ones future landlords actually check, do track eviction filings and will surface them even for cases you ultimately won. That second piece is the real reason to fight and resolve a case cleanly rather than just let it lapse.