>> Is there a recommended scenic route for Interlaken to Zermatt
If you travel via the Lötschberg-Basistunnel (the fastest route, which the timetable will bring up by default), it will take up to 2 hours 48 minutes. However, by taking the tunnel, you miss out on the scenery.
To take the more scenic Lötschberg-Bergstrecke (mountain route), put "Kandersteg" in the via field of the online timetable. That journey will take you an hour longer, but the mountain scenery is lovely.
>> and then Zermatt to Laussane?
There's really only one practical route from Zermatt to Lausanne, which the Swiss Timetable will bring up by default. You change trains in Visp.
The sector from Zermatt to Visp, is part of the Glacier Express route, along which you will have already travelled to get to Zermatt, and that sector is very scenic. If you sat on the left on the way up, you could sit on the left again on the way back to see the views on the other side.
From Visp to Montreux, there are larger towns and cities with modern buildings and commercial and industrial activity featuring fairly prominently, but is a pleasant enough journey nonetheless. Between Montreux and Lausanne you will travel close to the shore of Lac Léman (Lake Geneva) and past the Lavaux Terraces, which are lovely. However, the vines will probably be bare by then, somewhat reducing the scenic appeal compared with times of the year when their leaves are green or gold.
>> Or does it not matter in Nov/Dec time as the weather will not be great to see any scenery?
You never know with the weather, regardless of the time of year. It will be cold in November/December, but you could equally experience a clear sunny day, a foggy day, or pouring rain.