Actually I checked into this at one point. There's a knobby tire they put on MOST mt bikes - and it's continuous rubber in the center of the tire - so that you aren't bumping over the knobbies - but you do have knobbies if you need them. So they ride like a smooth tire.
I remember the guy at the store saying that you probably won't do much better with a smoother "road" tire.
But of course - big bumpy knobbies which don't have continuous rubber in the middle are a pain in the butt overall - a bumpy ride for sure. They're good on gravel through.