A “Saver Day Pass” is an “all you can eat” pass, good for unlimited travel on all public transit in Switzerland. As such it is not for “Zurich to Zermatt”. It is for anywhere to anywhere and back as often as you like.
You selected Zurich to Zermatt on the website, but you got offered a pass, not a ticket, because at the time of purchase that was probably the cheapest option available for your travel date. It is a good choice for a long trip.
As such you are not limited to what you selected. You can travel at any time, on any train. And you do not need reservations. Trains in Switzerland have enough seats. If you do want to sponsor the SBB however you can get seat reservations here:
https://www.sbb.ch/en/tickets-offers/tickets/point-to-point-tickets-switzerland/seat-reservations.html