The ultimate origin of Palpatine isn't exactly cleared up in the PT, anyway. Was he human and got scarred, or is his "scarred" skin his true appearance, unveiled? Screen canon doesn't say.
My understanding was that Palps was using his powers to retain his age and when injured by Mace's force lightning he released some of that power and it manifested itself with the deformities.
But I know what you mean... we used to freeze frame VHS tapes of ROTJ and wonder what the hell he was supposed to be under that hood.