I feel like the author didn't quite grasp the idea of transience granting beauty. One might argue the author did it on purpose, and thus it was the character of the late duchess who didn't understand it and wrote whatever. However, that wouldn't really make any sense, seeing how this is a work of fiction where the plot needs to be driven forward relying only on the limited medium. In real life, a lover could express something hilariously poorly, yet their partner would find it the most beautiful memory ever. In fiction, though, readers can hardly care about the duke's dead wife, who hasn't had any direct role in the story, being dead, so from a story writing point of view, it would be a questionable decision to use flawed ideas to give her depth.