i think men are allowed to use insanity, stupidity and vacantness, bumbling, and other potentially negative traits for their comedy and its generally accepted that they are largely putting it on for comedy value - if a woman does it, many people struggle to believe that it is just a comedy persona, and think that its the real person.
the most succesful female comedians and comedic characters are sharp, quick and intelliegent and formidable, whereas with men -although a lot are the same as that - we have people like bill bailey, alan davies, paul kaye, norman lovett, alan carr, justin lee collins, jeff green, eddie izzard etc.
things like smack th pony and titty bang bang have enjoyed some success but i know a lot of people don't like to see women acting 'gross' and daft so they haven't been huge