Here is the pertinent part of the interview with Ms. Watson:
‘I find this whole thing about being 18 and everyone expecting me to be this object… I find the whole concept of being ‘sexy’ embarrassing and confusing. If I do a photo-shoot people desperately want to change me – dye my hair blonder, pluck my eyebrows, give me a fringe. Then there’s the choice of clothes. I know everyone wants a picture of me in a mini-skirt. But that’s not me. I feel uncomfortable. I’d never go out in a mini-skirt. It’s nothing to do with protecting the Hermione image. I wouldn’t do that.
‘Personally, I don’t actually think it’s even that sexy. What’s sexy about saying, “I’m here with my boobs out and a short skirt… have a look at everything I’ve got”? My idea of sexy is that less is more. The less you reveal the more people can wonder.’
In this quote Ms. Watson makes an argument for modesty and the justification for modesty is that being modest is more sexy than being immodest, a similar sentiment is contained within the "Modest is Hottest" slogan. I'm aware of two definitions of the word "hottest" as it is used in this context. The first would be claiming that modesty is the most popular. It is obvious that this interpretation is simply inaccurate. The second definition would be the claim that modesty is more sexy.
The concern that I have with Ms. Watson's comments and the "Modest is Hottest" slogan is that both seem to be promoting modesty as a way to achieve greater sexiness. Both seem to recognize the negative effects of immodesty on girls and society but they ignore an underlying driver for immodesty which is a desire to be sexy. Women are constantly bombarded with messages saying that they must be sexy and a woman's value in society is largely based on their ability to arouse men's sexual desires. We are taught that a sexy women are valuable, loved and powerful. We as a society have so deeply accepted the role of a woman as a sex object that even when trying to promote modesty we do so by arguing for modesty as a means to achieve the goal of being even more sexually attractive. I do not believe that the purpose of modesty is to be sexy in fact I think the purpose of modesty is to not be sexy. This is not to say that a modest woman can not and will not be attractive*, but her attractiveness, personality, intelligence and kindness will be allowed to show because those around her will not be distracted or repeled by her immodest exposure. When we argue for modesty as a way to achieve greater sexiness any gains in modesty will be shallow and short lived. Furthermore we only re-enforce the incorrect and damaging view that a women's worth is defined by their sexual appeal.
When arguing for modesty we should not just argue for better behavior. We should instead expect better behavior by instilling in women a greater understanding of the true value of women and womanhood. As society learns to truly value women and not just their bodies then there will be no need for immodest clothing as immodesty will impart no perceived value nor power.
The interview with Ms. Watson is found here. Warning many of the pictures of Ms. Watson in the interview portray her in immodest clothing.
* A couple of notes. First I am not claiming that a woman should never be sexy, nor that a modest woman will not ever be sexually attractive. I'm arguing against the overt predominent sexuality promoted by immodest clothing. Secondly note that I am distinguishing between being attractive and sexy, unfortunately society unnecessarily conflates these two attributes.