Normandy and Mont St Michel are not near to DLP. It's probably 4.5 or 5 hours driving from DLP to Mont st Michel. France is a big country compared to the UK, but probably won't seem big if you are coming from the US.
If travelling from the UK I would opt to take car and ferry to Cherbourg or St Malo, you could then visit the cemetery, towns such as St Mere Eglise, the Normandy beaches and Mont St Michel. Then drive to DLP and possibly then take the shorter Calais crossing back.
If you're flying to France then I think I would definitely hire a car. This will give you much more flexibility, as there are lots of smaller places you might be interested in visiting. I'd also allow at least one overnight in Normandy, probably two.
If you're interested in WW1 history as well as WW2 there are other places near (ish) to DLP such as Compiegne, Vimy Ridge (about halfway back to Calais), the Marne and Somme monuments, Ypres (Ieper) and Sanctuary Wood, which might be worth adding on to your trip.
Trains from Marne la Vallee to Pontorson for Mont St Michel require two changes and take about 5 or 6 hours, it's then about 20 minutes by car to Mont St Michel.
The nearest train station to Colleville sur mer (where the cemetery is) is probably Bayeux. Again from Marne la Vallee it's a couple of changes and takes 6 hours.
There is a direct train from Pontorson to Bayeux which takes a couple of hours. The voyages-sncf website has train times for France.
So it's possible by train but I'd say a car would probably be cheaper and more convenient plus would give you options to visit other places.
