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! “
It is not a chatzitzah, and IMO any mikvah lady who raises her eyebrows in an obvious manner at a young tattoed baalas teshuvah (who is coming to fulfill the mitzvah of Taharas Hamishpacha because she is NOW A FRUM WOMAN), should be given mussar immediately for potentially causing embarrassment to the young woman. What an aveira were she to make that B”T feel awkward, judged,or uncomfortable. The girl could chas v’sholom refuse to go back to a mikvah again.