This verse is difficult to interpret because the meaning of some of the Hebrew words are not clear. The general consensus is that the mistress refers to the cult statue (possibly Ishtar, the goddess of beauty).1 It was common for invading forces to remove cult statues from conquered cities as a sign of total defeat. The Assyrians would have done the same thing when they invaded cities.
The mistress may also refer to the whole city that is carried away into exile by the Babylonians.2 Even if it does refer to the city and not to some cultic statue, the end result is the same: the city of Nineveh has been utterly humiliated in complete defeat.
7 its mistress is stripped; she is carried off, her slave girls lamenting, moaning like doves and beating their breasts.