Why is this?
I understand the appeal of actually being more intelligent than most people will assume you are, but I can't see any benefit in actively promoting yourself as a bimbo... yet I see it quite often.
Do any of the females here know anything about this phenomena & why it is?
It is doubtful there are any women who are threatened or turned off by a man being more intelligent or better at humour than they. Women are less able to deal with being the object of amusement on a personal level than many men are though. It could be that when a man laughs at what a woman said, she is much more likely to take offence, but if a woman laughs at a man it will cheer him up as long as its not meant in a nasty way.
Eg.
Woman falls off her bike doing a somersault - man laughs because it just looked funny and she's okay: result = woman is not pleased with man
Now do a sex reversal. Most men (if they find the woman remotely attractive) will actually be pleased to have made her laugh. That's one of those really sweet things about men!
In the case of intelligence. I know I am very typical in this: I would not have a long-term relationship with an intellectual inferior no matter how talented, rich, practical, handsome,etc in preference to someone at least on my level and preferably more so.
Men are more than happy, (with some sensible ones being the exception) to go with a woman who is very stupid but okay looking.
Its the rule (with some exceptions). Perhaps there is an evolutionary reason for this: the man has to provide for the woman and so being intelligent is more necessary and a woman who thinks her man is dumb is going to be trying to over-rule him constantly. (Some women who go for low intelligence men, only do so because they want to boss them.)
Intelligence may be somewhat sex-linked due to these choices of mate. Perhaps a woman, while being slightly less intelligent than her man, may yet have the genes to pass on a higher intelligence than she has herself to her sons. Her genetic intelligence is suppressed by her being female just so that she can be matched with a man who seems smarter. That's just a pet theory of mine.
Some women may play bimbos just so they can be found attractive by a wider selection of men.