Floisme: Admittedly I've only looked at still photos so maybe I'm missing the mannerisms.
Yes you did, it was all about the mannerisms. Not the frock she was wearing.
As to Amal Clooney's mannerisms, that made me think quite hard. If gender is culturally mediated by social expectations, as feminism posits, is there such a thing as neutral gender expression?
Her mannerisms might be read as "girly", but how much of that is our understanding of cultural expectations versus her own personality? What would be appropriate mannerisms for a woman of her intellect and capability? Would more stereotypically masculine mannerisms seem more suited to her achievements? But that would be playing into another stereotype that success depends on behaving like men.
Dance, her mannerisms were, and there are not really adequate words to describe really, a complete and utter embrace of the femininity display of acquiescence. Perhaps a poor choice of words to say she 'lost IQ points', however, to all the males viewing that display, she did. I am not sure why women cannot see that, perhaps not working in male dominated fields?
Yes, there is totally a way you can conduct yourself in a professional manner, without displays of femininity, or indeed masculinity - but you have to also be aware of what those displays are (in order to avoid both). You can get a start by the clothes you wear (I am talking about in work, not social function, although I would never wear a dress), and not be giggly (be serious), not be apologetic for having an opinion, certainly be engaged and not aloof, but also connect with the inter-personal dynamics of the meeting, without being 'the waitress' type personality. It does help being three times as good as any bloke in the room, but seriously, that is not hard to achieve at all.
It is a mistake to say one must be 'an honorary man', you don't have to be. But avoiding femininity displays is a must if you are to be taken seriously at all. If you do partake in femininity displays, you will be judged (by men) by those displays.
Women make the big mistake of thinking some of their 'harmless' pretty (femininity) displays/clothing is no big deal. It actually is if you want to get ahead, and be taken seriously within a male-dominated world (and all of it is btw). How men view these displays, is that they will see you as 'less than human' (ie less than male), and treat you accordingly.
TL;DR Yellow dress? I did not notice. I did notice her demeanour though. Which demeaned her. I am sure she is better than that.