Well, melatonin helps you get into the deep sleep stage. The deep sleep stage is when the most skin repair happens. I am not sure I get into the deep sleep stage since I frequently wake up to urinate. There are other natural supplements that can also help you get into the deep sleep stage.
Since it is harder for melatonin to reach skin through the bloodstream, I would directly apply the liquid instead of take capsules. But you can also take the capsules to go into deep sleep stage if you are not getting there.