I think the point and intention of Graft's "favor" was to explicitly [i][b]not[/b][/i] have either Salem or Light make [i]any[/i] comments about her appearance, at all... xD
After all, if they do, Kath's library would record it, and for whatever reason, Graft doesn't want this to be leaked to the rest of the Chateau.

Naturally, Light has no idea or inkling of this, so it would be more in-line with her persona to disregard this favor, but in case you were unsure of [i]why[/i] this favor was asked, I think the above is the reason.

At least as far as I understood the post. ^^;