It is a little unfair to expect that a mikva lady not raise their eyes, inadvertebntly, when seeing the tattoos.
The reason many people have tattoos is specifically for that reason, to cause others to raise their eyes. I am sure the BT at this point would not want it, however, the tattoo is permanent and the impact it was designed to make does not just go away.
It is likely/possible that the tattoo that raised eyebrows was not just a small little mark, It may have been one of the “elaborate” ones in areas were people do not usually expect to see such marks.
There are certainly mikva ladies who are more able to control their reactions, and those who are better at welcoming all women to partake in the mitzva. However, it is like everything in life, not everyone is at the same level.
It may be a little bit overly judgemental of you to exect that every mikva lady, even very good ones, can be so in control of their reactions.