The challenge to cleaning a stucco ceiling, you'll soon learn if you tackle this project, is to get the ceiling clean without dislodging or disfiguring its stucco. This will mean you'll need to avoid using liquid cleaners and stiff brushes.
Much of what you'll be able to remove from the stucco are spider webs, dust, and loose particles that have collected there over time. To do this, use a vacuum cleaner with a soft brush attachment. Keep the attachment opening far enough away from the ceiling that its suction will not loosen or detach any of the stucco.