Your pass is valid in the bus, but I doubt reservation is possible.

SNCF claim in their timetable data some bus lines have mandatory reservations when in fact they don't. I haven't used this particular bus, but found it to be the case on a few others.

Thanks for the adivce!  My worries is that the bus will be very full.

When are you travelling exactly ?, anyway the train to Villedieu les poêles has a reservation mandate, so SNCF know how many people are on that train, and can adapt the bus offer with that information. You should be fine

Hi I am planing to visit Mont Saint Michelle too. From Pontorson Mont Saint Michele train station there is often a bus line to mont saint michele. But the current timetable is valid only till this sunday. The next one is not published yet.

Not included in Interrail the bus line from Pontorson.

Sure about that? SNCF sell tickets for it.

I think there is a TER bus 2 times a day (included) and a local bus line the runs more often (not included)

That could be the case. On the TER Normandie website, the bus in connection with the morning train from Paris is a "Car TER”:

That means passes are valid.

Do I need to book Car TER 36873 or I will buy the ticket on board?

You need to add it to your pass before you board. And then your pass is your ticket.

You could ask at a ticket office in France if they can book a reservation for this bus, but I doubt that will be possible.