Daas Yochid & Sam,
As Sam wrote properly done nail polish (as opposed to chipped or old) is certainly miut v’eino makpid and not a chatzizah m’ikkur hadin. And, as Daas Yochid wrote, Ashkenazim (following Rama in Yoreh Deah 198:1) are makpid not to be tovel even with things that are not a chatzizah. Arguably this includes nail polish.
However, in Yoreh Deah 198:17 the Shulchan Aruch writes that ????? ????????????? ?????????? ??? ????????? ????????? ???????? ???????, ?????? ??????. And the Rama does not argue but rather adds that the same rule applies to shochet whose hands are blood stained because the norm of shochtim is not to be makpid. The Rashba gives two reasons for being meikil by hair and makeup: 1. the woman specifically wants them so they are even better than miut v’eino makpid and become part of guf (halachically); and 2. they lack mamushus. The Rashba has to give these reasons because hair dye is not a miut but ruba (we consider hair as if it’s own unit).
Therefore, even Ashekenazim are meikel by hair dye despite the Rama in 198:1. Presumably, because it is given a din of the guf itself. If so, then Ashkenazim should also be meikel by nail polish because the woman specifically wants it there and it becomes part of her guf. (On the other hand, nail polish has more mamashus than hair dye, so arguably the second reason of the Rashba doesn’t apply).
Regardless of the actual halacha, it is very uncommon for ashkenazi women to wear nail polish to the mikvah (and many mikvaos or mikvah ladies don’t allow it). So it remains a reasonable explanation for why Yeshivish women do not put on nail polish.