I think trying to decide if a muse is appropriate for an artists is utterly ridiculous. A muse is something that inspires and it's rarely something unsensational or lacking in scandal, especially in fashion, photography and art. Who are we to say whether or not it's "right" if an image of Zahia draws up a sketch in Karl's mind that ends up being a piece of clothing we adore? Ridiculous. It would be harder to find great artists that DIDN'T have inappropriate muses than it would be to find those who kept their muses purely tasteful. Historically they have been mistresses, prostitutes, dancers, actresses and lovers, Zahia is just another in a long line of women who have filled these shoes for the greats of their eras.
And let's look at the fact that he could have a lot less "appropriate" muses, like say those who haven't designed two lines of lingerie that showed at Paris HC week before the age of 20.
Furthermore, from a purely aesthetic position, even though she's kind of a butterface ("but her face" for those of you not that great at english word games) she has a body that is anatomically incredible. Regardless if you think her boobs are real or any of that jazz, can your back bend that far? Is your *** that pert? Your legs that long? No? Then Karl Lagerfeld probably won't use you as an "inappropriate" muse.