Sagging ceiling drywall always indicates a problem that needs correction, and it adds a touch of shoddiness that no room needs. If it is sagging because it got wet, it probably means that a water pipe, the roof or a wall is leaking. If the drywall is sagging between joists, or pillowing, it means the installers probably used 1/2-inch drywall when they should have used 5/8-inch. The weight of the insulation is causing the sagging, and the chances are that the drywall will eventually start falling unless you do something. In both cases, the remedy involves hanging more drywall.