Tattooing is a hidden skill. This means that they don't have a level that you'll see, but they'll get better at it as they give tattoos. To start with, they'll end up covering sims with really bad tattoos, but after some practice they'll be able to give good ones reliably. Likewise, they only get $75 for a session in the chair, as opposed to the $150 that the artist at the salon gets.
Unfortunately, the only way to make any money from it is to constantly invite sims over and tattooing them when they visit (you can't give tattoos at the salon). Also, you don't get paid for giving tattoos to members of the household.
