In practical terms, this really has nothing to do with the question of how transformative a tattoo is or is not as a derivative work.
The reason this hasn’t been a problem before is simply due to the nature of a tattoo - people typically only see the finished product after it was produced, long separated from the tattoo artist that inked it. Even with celebrities whose images are all over the media, a rights holder isn’t going to go track down Vin Diesel‘s tattoo artist to demand compensation.
There have been high profile tattoo artists, of course, but one assumes they have been aware of their visibility and thus by and large steered clear of exactly what Kat von D just stepped into.
In 2011, the tattoo artist who did Mike Tyson's tattoo suing Warner Bros over The Hangover 2 / Ed Helms. It was settled out of court so no precedent was set, sadly.
The reason this hasn’t been a problem before is simply due to the nature of a tattoo - people typically only see the finished product after it was produced, long separated from the tattoo artist that inked it. Even with celebrities whose images are all over the media, a rights holder isn’t going to go track down Vin Diesel‘s tattoo artist to demand compensation.
There have been high profile tattoo artists, of course, but one assumes they have been aware of their visibility and thus by and large steered clear of exactly what Kat von D just stepped into.