Lumiya was known as Dark Lady, but I don't think she ever took the title of Darth. Zannah and Cognus were both females and they took the title of Darth. Then there was Traya/Kreia who took the title. Darth is gender neutral, kind of like being a Jedi Master. There are plenty of women Jedi Masters that don't refer to themselves as Mistress when that would be the "P.C." term.
This is a good point. But I was thinking more along the lines of count and countess, regent and regess. You get the picture.
But good point about Masters. Female Jedi that reach that rank are not referred to as Mistress, which would sound tawdry.