Well, it kinda is. You're basically selling a part of your body for a few BTC's.
Also, based on where you proposed the tattoo should be placed, there's no guarantee that it would even be advertised.
I mean, if it's on your back, you wouldn't be going out to a mall shirtless, would you?
There's also no guarantee that you won't cover the tattoo when you're out on the public.
But hey, it's your body, so do whatever you want with it I guess.