Where a Mobilis Day Pass is valid depends on how many zones you buy it for. Please note that it covers only trains, buses, metro and funiculars - not the boats on Lac Léman (Lake Geneva).
If you want to use the Mobilis Day Pass between Lausanne and Chillon Castle, you will need a pass covering 8 zones (Zones 11, 12, 19, 64, 70, 71, 73, 77), which costs CHF 29.60.
There is another pass called the Lavaux Riviera Pass, which costs CHF 46.20. The Mobilis Lavaux Riviera Pass day pass is valid within Mobilis zones 11, 12, 19, 64, 70, 71, 72, 73, 74, 75, 76, 77 and 78, as well as on the boats between Lausanne and Villeneuve, until 22 October 2017. After that, boats are not included, and I think the pricing changes.
This is a different pass from the similarly named Lavaux Pass, which only covers zones from Lausanne up to Vevey and does not cover the zones in which Montreux and Chillon Castle are located.
There seems to be little information in English, but here are some links that might help:
Download Mobilis Zone Map (right-click on map graphic to download PDF zone map)
Statement that Mobilis tickets are not valid on boats
Mobilis tariffs (NB: Boats not included)
Cartes journalières plein tarif are full-fare day passes.
By the way, what other plans do you have for travelling in Switzerland? Have you considered getting a Swiss Travel Pass?