Don't stay on Mont Saint-Michel if you want a good night's sleep.
The hotel staff were friendly, and the room was clean and comfortable, so I have no complaints about the accommodation itself. The problem was the noise.
As the hotel is on the main street, visitors were outside until nearly midnight. After that, hotel staff stood outside talking until around 3:00 a.m., and from 5:00 a.m. the bin collections and restaurant deliveries started. Sleep was almost impossible.
Given the premium price of staying on the Mont, it was a very disappointing experience. My advice is to stay on the mainland, visit the Mont for dinner and the abbey, then return to your hotel for a peaceful night's sleep. You'll enjoy the experience far more.