Q. If one is makpid to get a haircut only on days when doing so is a mitzvah (erev Shabbos and yom tov), may one do so on erev Yom Kippur? I have not seen anywhere that getting a haircut Erev Yom Kippur is a mitzvah, unlike erev Shabbos or other erev yomim tovim.

A. Although taking a haircut on Erev Yom Kipur is mentioned in regards to an ovel (see Rambam H. Ovel 10: 8), indeed it seems to be disregarded for others.
Horav Shlomo Miller’s Shlit’a opinion is that most people would cut or trim their hair before Rosh Hashana or before Sucos, as mentioned in Shulchan Aruch. Thus it would be uncommon and unnecessary on Erev Yom Kipur, when time is of the essence. However, if someone needs a haircut, or in the case of a Cohen Gadol, who trims weekly, he would do so on Erev Yom Kipur too.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a