My take on this is:
(i) It's something like the Biblical quote "it is what comes OUT of a man that corrupts him." Palpatine is all fricked up because all that evil is coming through and out of him. So it fargles him up (because he has embraced it), but Luke is innocent, so all it does is give him pain, because he's like an opposite polarity.
(ii) It's not just zapping him, it's draining him because it's leaving his body (I mean, okay, he also got zapped by his own electrics, but I think it's more a matter of having to put up a struggle to break through Mace's desfenses than backwash which is disfiguring him; rather - he is being drained. He may be lying about the extent of this, but I still think it holds true).
(iii) It could also be that his power was helping sustain a false youthful benign "good" appearance, hiding his true evil ugly shape, a bit like a Portrait of Dorian Gray kind of thing.
(iv) I need a job.