Language, WordMan. Language.
Although, in truth, “beautiful” is a subjective term, for ease of communication there are contemporary standards that may be applied.
It is of course always acceptible to say “I think so-and-so is beautiful” or “so-and-so represents my personal idea of beauty” no matter what the particular so-and-so looks like. However, when speaking in absolutes like “Could Francoise Hardy be more beautiful?” your audience will manufacture images based on contemporary standards.
To illustrate: I personally find Maura Tierney to be infinitely more attractive than Cameron Diaz. But I am able to recognize that I have quirky tastes. If I were in a general conversation about feminine pulchritude I would hold up my personal ideal in contrast to contemporary standards.
In this Thread you have promised an example of beauty, your promise was made in such a way as to suggest that the example you have to offer will transcend personal tastes. You then provide an image of a woman whose beauty has unique appeal. You’ve set certain expectations then failed to meet them. That’s why the responses have been less than enthusiastic.
Personally, although I never heard of her before I read your OP, I agree that, as represented in the photo you provided, she is quite a sexy attractive woman. I would not have used the word “beautiful”, because that is a word that implies an absolute- or at least that popular standards have been considered while measuring (again, you can certainly always use the term “beautiful” when prefaced by “I think” or “IMHO”). If I were moved to start a Thread about Francoise Hardy I would probably have said “Francoise Hardy is very sexy in a dark, alluring, aloof kind of a way”. I think such an OP would have gotten more sympathetic responses.
She does, after all, have a rather mannish jaw. A feature that will be a turn-off to some. It doesn’t bother me personally. I still think she’s very sexy and would compare her to contemporary singer-songwriter Quincy, who also has a very mannish jaw but is still very sexy.
Also, you could have found a better photo of her. Something like this might have made your case stronger. This one’s good too. Or this or check out these (I particularly like the second image down on the left). I found all of these from a simple Google image search.
I particularly think she has aged to be a very sexy older woman as evidenced by the photo on the homepage of her website.