My first thought was that the tattoo might qualify as a chatzitza at the mikveh. On further reflection, I realized that anything– including a tattoo– that is a permanent part of a woman’s body is not a chatzitzah.
I believe it is, however, assur regardless for reasons already mentioned. And incidentally, I know a few baalos teshuvah who have tattoos from before they were frum…and apparently, they’ve raised some eyebrows at the mikveh!