It’s comforting to know that the author of the "Juliar Bob Browns Bitch" placard, one "Tim", didn’t think it was offensive to the prime minister. However, one shudders to think what level of vitriol would be required to pass the gentleman's threshold of offensiveness.
It’s possible of course to say harsh things about a female politician without being misogynist. It’s even possible, I’d suggest, to use the word bitch in a non-misogynistic way, as a sort of equivalent to the definitely male-only bastard, or perhaps even in reference to behaviour that, whether by a man or a woman, might be considered bitchy. Though not, clearly, if the intent is to compare to a female dog. When I grew up in the 70s and 80s, "dog" along with other such charming terms as "bushpig", were standard terms of abuse of girls.