Seat reservations are not a thing in Switzerland: they are available on some trains but nobody makes them except few tourists (lived here all my life and never made one).
The country works like a giant subway system. People hop on, hop off and sit wherever they want. Trains are long and capacity is plenty. If a carriage is busy, simply walk to the next one. Don't overthink it. :)
Zermatt-Visp is a regional train. Since Zermatt is a termini station people will board from the rear. Go to the front carriages.
Visp-Lausanne will be empty when you board. It gradually fills up along the way.
Lausanne-Paris TGV high-speed trains indeed have mandatory reservations (29€). France has a lot of mandatory seat reservations (due to infrequent trains/different passenger mentality).
Some info here: https://www.seat61.com/interrail-and-eurail-reservations.htm