Elizabeth I of England and Catherine the Great of Russia were both pretty dictatorial: ruling on a whim, appointing their favourites to whatever role they wanted, and imprisoning anyone who looked at them funny. We often say that Catherine's ministers ran the country for her, but the same was true of Mussolini and nobody's claiming that he wasn't a dictator.
I think you could argue that this is a case of the boys' club situation: women in traditionally male fields feel pressure to act in very macho ways in order to reassure the men around them that they're suitable for the role; and absolute rulership is absolutely a traditionally male field.
However, I can't think of any women who seized dictatorial power in the same way that Mussolini did, yeah.
ariadne