The altar should never, I mean never, ever, ever, be hidden by any decoration and certainly not the Nativity scene. People go wild hiding the altar and ambo with flower and decorations placing these directly in front of these two important liturgical furnishing. Decorations should frame, not block the altar or ambo or hide it.
Never, ever, ever, was the nativity scene placed in front of the altar for the Tridentine Mass prior to Vatican II or after or for Ordinary Form Masses ad orientem! Why, you ask? Because you can't get to the altar for all the clutter of poinsettias and the creche! No one did that for over 1,500 years!!!!!