Is Montreaux to Zermatt a day trip?

Yes, what you do is look up the weather forecast the night before and verify it when you get up and if it is not snowing raining foggy or cloudy then take the train there. That's the best way to go there if not staying.

Also if you catch the 6:03am train you will be a little ahead of the crowd.

It is 2 hours and 40 minutes each way, so get an early start, like 6:30 am if the weather looks favorable.

For someone who considers train travel in Switzerland exciting/awesome/memorable as a part of the trip itself (without luggage of course), I can justify almost any 2-3 hour Swiss train journey, so yes. But as always, it depends on how many days you are in Switzerland, where you are already staying, and how many times you have been to Switzerland before.

One example - if someone was only staying in the BO Region for a few days and had a choice to either hop on a train from Lauterbrunnen for 3 hours to Zermatt to see that for a few hours and then hop on another train back to Lauterbrunnen OR they can take the gondola/train up to Murren and walk to Gimmelwald then take a gondola down to Stechelberg to then walk the entire Lauterbrunnen valley with a stop to see Trummelbach Falls then there is no choice IMO = option 2 is much better time spent.