I always thought that the reason the Yeshivish world doesn’t wear nail polish is because it requires time and effort to remove it properly for tevilah and therefore a woman would not wear nail polish in the days leading up to tevilah.
Now, because they won’t wear it leading up to tevilah, putting on nail polish is indication that the woman has recently gone to the mikvah and publicizing that fact is viewed as untzniusdik.