On second thought, I think a tattoo could help with a woman who is attractive but not stunning. Maybe a woman who is a 6 or a 7. Beyond that, the tattoo distracts from their beauty.
I noticed this with the woman on the cover of The New York Times magazine (Emily Gould). She is average looking; cute but not overly stunning even done up for the cover piece. The tattoos enhance her some, even if they are a bit much.