No — as of now, Kirk Cousins is expected to be the Week 1 starter, not Fernando Mendoza.
The Raiders signed Cousins on a flexible 1-year deal specifically to be the bridge starter while Mendoza (the No. 1 overall pick) learns the Shanahan-system offense under HC Klint Kubiak (from the Kirk Cousins page). The team's entire approach is "patient developmental" — they want to bring the rookie along slowly rather than throw him to the wolves (from the Las Vegas Raiders page).
That said, it's not fully locked in. A May 26 article notes "Raiders QB Kirk Cousins Not Certain To Start In Week 1", so Mendoza could push for the job if he lights up training camp. But the organizational plan — with Tom Brady's mentorship, a punishing 7th-hardest schedule, and the investment in Cousins — points to Cousins starting early in the season with a mid-year transition to Mendoza as the likely outcome. Sports Illustrated called it "the wise decision to bring him along slowly" (from the Fernando Mendoza page).