What an interesting question Reina! It will be interesting to see what other members have to say -- the comments posted already are quite interesting.
My feelings are that while our society can be heavily focused on physical appearances (how a princess styles her hair, her make up, her jewellery, her shoes, her purses, how she dresses and which designers she wears), the princess herself shouldn't be caught up in this. How a princess looks when she goes out to perform her royal duties is what sells magazines -- but what sells royal families to the citizens who are supporting them through taxes is the work they do and how they represent their countries while at home and abroad.
I think physical looks are over rated anyways. Of all our 5,000+ members I doubt that we could all come to agreement on which of the Crown Princesses is the prettiest or the best dressed. Looks are relative and personal. Some members may find one princess pretty because of her glamour while others may like another princess for her wholesome nature. And of course every country will say that their Crown Princess is the prettiest -- it's natural to have pride in one's royal family.
While I certainly want my royals to look professional when out working, ultimately it matters more the kind of work they are doing and how hard they are working. A princess may dress like she's just stepped off the cover of VOGUE magazine but if she only performs two or three royal duties a month that wouldn't impress me much. Whereas even if a princess looked rather on the plain side (but still professional) but was out doing 20 royal duties a month and representing a variety of causes I would find that much more admirable.
I absolutely agree with Freedom that to whom much is given much is expected. Royal families are bestowed with tremendous status, prestige and luxury -- as well as tremendous obligations and burdens. But the mark of a true royal is someone who rises to the challenge rather than hides in the palace or hides behind good looks and nice clothes.
As a personal example, I have criticized on more than one occasion my loathing of Victoria of Sweden's ponytail and her rather plain style of dressing. But despite the ponytail, the glasses always, and the lack of colour in her wardrobe, I do admire very much how hard Victoria works, how much she sacrifices, and how she seems to strive endlessly to represent Sweden with the utmost professionalism and dedication. I wouldn't trade (and not to be presumptous, but I don't think the Swedes would either) Victoria in a couple of Nathan suits, Prada and Versace gowns for this commitment and dedication.
Letizia is well known for her recycling of her wardrobe, and as much as I grumble about seeing a blue jacket for the umpteenth time I do appreciate all those upteenth occasions she has worn it.
The mark of a good princess isn't by how vast her wardrobe is or how well stocked with designer clothing it is -- but by how much she cares about her country, its citizens and her dedication, commitment and sincerity in carrying on the role of Crown Princess.