Will this photo cell work properly if it's mounted on a ceiling facing down? I'd like to use it to control a flush mount ceiling light on our porch.

Physical position is irrelevant. The amount of daylight governs the function. Mounted on a porch ceiling it may "see" a low light level earlier than "normal" in the evening and turn on too early, or it may "see" a low light level in the morning and turn off later than normal, or not at all. Once a photocell cycles into the on mode it takes a decent amount of daylight to cycle to the off mode. A photocell hiding behind the bushes will usually cycle off late or sometimes not at all. A photocell only fails at night when it is defective. Eventually it always gets dark enough for the photocell to cycle on!

No, this will not work on a ceiling facing down.