Not at all. It was built in the Middle Ages and you pretty much get the same experience as people back then. There's a first ascension up the hill in a tight crowded street that takes about 20 min. After that, you hit a very steep set of stairs that gets you to the ticket counter inside the main abbey. You then begin your tour that takes 1-2 hours visiting a place built on multiple levels on the sides of the mount. Except the main church, built on top of the rest of the abbey, pretty much every room involves a set of stairs to reach it. At the end of the tour, you need to go back down the same steep stairs and winding road.
I love the place, having been there 5-6 times, including 3 times up the flèche at the very top, something offered only a few days a year. I even spent one night in one of the hotels on the Mont. I would not recommend it to anyone with mobility issues. He will probably give up before reaching the main part of the visit.