Shouldn't be allowed in my book.
Anyone who disrespects their own body enough to have permanent graffitti embedded in their skin should not be allowed to fly a jet liner. If they are prepared to mutilate (and yes it is mutilation) their own body, will they respect their passengers body's any more?
Don't hit me with the "it's artwork" and "people have been doing it for centuries" rubbish, we live in a modern civilised society of which self mutilation plays no part, the best a person who indulges in this sort of behaviour can be described as is ferral!