Never said it was about flight safety although one could argue that piercings are a case of flight safety as they quite often cause injuries during evacuation training.
It is of course about appearance to the paying customer. Many of them don't exactly like goth cabin attendants or punk pilots. That said we are of course free to pierce areas of our bodies that are not usually seen (the tongue is deemed visible) and wear tats where it is usually covered by the uniform. Going on a discovery voyage after work can be a very pleasant pastime...