Definitely, but its not unique to just derm.
In every facet of life, being love, career, or asking a stranger for the time, better looks = more success.
The reason there are a disproportionately high number of sexy female dermatology residents is likely because a disproportionately high number of sexy female medical students apply to it, seeing as how its the holy grail of medicine and pays beautifully, no pun intended.
On the opposite side of the spectrum are things like pathology, where the lack of sexy females isn't because looks don't help, but because very few sexy females apply for it. Radiology used to be like this until it became bank, and now the best looking department at many schools is the rads department.
I can tell ya, as a male resident roaming the hospital, I see tons of medical students doing "away" rotations here. The ones doing plastics, rads, uro, derm etc could basically just quit and become lingerie models. The others are mostly regular looking folk.