Hello Tiqueengouch70
Thank you for your request.
I apologise for the late answer. Yes, in Switzerland reservations are only obligatory on certain trains. (e.g. panorama trains) Seat reservations are not available on most IR trains. You do not need a reservation on the IR 90 or IR 95 trains for your journey from Sierre to Geneva.
Have a good day.
Best regards, TatianaR