People form opinions and draw conclusions about us based, in part, on the choices we make regarding our appearance. Sometimes the conclusions are in error. For example, one may see an immodestly-dressed girl and conclude that she is promiscuous, when she is not. An employer may believe that an applicant without tattoos is more likely to show up on time, sober, and not steal. Due to the coincidence of tattoos with drug use and prison, this is not an unreasonable belief, although it may be inaccurate in the individual case.
As many comments have observed, tattoos are more common and more admired by younger people. That fact could explain some of the correlation of tattoos with lower incomes, because younger people, on average, have lower incomes than older ones.
Well for most part, the younger people aren't the ones doing the hiring.
“People form opinions and draw conclusions about us based, in part, on the choices we make regarding our appearance.”
And not without good reason. A slob who shows up with neck tattoos may be the best person in the world, but he’s not getting the job because of the vast number like him who are not. Piercings and gaudy bling aren’t a good selling point either.