Hi SwissTravel
On the Geneva-Visp-Zermatt stretch, it is not possible to reserve seats. Seat reservations are only possible in EC, IC, or PE trains.
You may download the SBB Mobile app. On the day of departure, look up the connection you desire to take. In the connection details, it is possible to see the average occupancy per car. Board one of the cars which indicate lower frequency.
Best regards,
Roland