I don't profess to know many tattoo artists, but the ones I have met have all had a talent in art and an interest in tattoos before combining the two to become tattooists. A lot of the websites of the good studios(eg 9 Boycess Street and Magnum Opus in Brighton) feature not only tattoos, but sketches, paintings and sculptures of the tattooists as well.
When doing a tattoo, the artist pulls/stretches the skin to make it taut. That's how they get straight/even lines.
I think tattoo artists and their line of work is very underrated and looked down on by society in general. The ones I've met are very interesting and talented people.