I would fold it up if the monorail is busy. When there is room, no need to fold. The monorail is convenient because you don't need to fold the stroller to get on. Plenty of room!
We were there for a week riding the monorail and never folded it up. It's great to be able keep the baby sleeping in their stroller while you ride the monorail.
I think it's just common courtesy to fold it when it is crowded. Not doing so is plain rude. If it's not busy or crowded, and there is room, then who cares...