In the movies, the Empire was shown as being completely devoid of females. Lucas did this intentionally, to show the infertility of such a top-down authoritarian structure. Naturally, to enhance their villainy, they were also racists (or more accurately, specist).
The expanded universe (comics, novels, etc) uses both aliens and females in the Empire to tell specific stories (especially the star-crossed lovers, and the falsified innocence twist). While our dubiously heroic lab mouse is distinctly a non-cannon species, it's still fun to put her in command of a Star Destroyer.