True Left

"Contrast this with the mythical Queen of Camelot, Guinievere, whose exceptional beauty was nothing short of a goddess:"

I think the whole point of the story is that Arthur falling for Guinevere (who later betrayed him) was a mistake (which ended up causing the fall of Camelot). So if we praise Guinevere's beauty, we would in effect be encouraging the mistake to be repeated! Nimue, in contrast, was the one truly trying her best to help Arthur all the way through (but Arthur was too stupid to appreciate it):

https://en.wikipedia.org/wiki/Lady_of_the_Lake

the Lady of the Lake traps Merlin in a tomb, which results in his death. She does this out of cruelty and a hatred of Merlin.[27] In Le Morte d'Arthur, on the other hand, Nimue is still the one to trap Merlin, but Malory gives her a sympathetic reason: Merlin falls in love with her and will not leave her alone; Malory gives no indication that Nimue loves him back. Eventually, since she cannot get rid of him otherwise, she decides to trap him under rock and makes sure he cannot escape. She is tired of his sexual advances, and afraid of his power as "a devil's son", so she does not have much of a choice but to ultimately get rid of him.[7]

After enchanting Merlin, Malory's Nimue replaces him as Arthur's magician aide and trusted adviser. When Arthur himself is in need in Malory's text, some incarnation of the Lady of the Lake, or her magic, or her agent, reaches out to help him. For instance, she saves Arthur from a magical attempt on his life made by his sister Morgan le Fay and from the death at the hands of Morgan's lover Accolon as in the Post-Vulgate, and together with Tristan frees Arthur from the lustful sorceress Annowre in a motif taken from the Prose Tristan. In Malory's version, Brandin of the Isles, renamed Brian (Bryan), is Nimue's evil cousin rather than her paramour. Nimue instead becomes the lover and eventually wife of Pelleas, a gentle young knight whom she then also puts under her protection so "that he was never slain by her days."

In an analysis by Kenneth Hodges, Nimue appears through the story as the chivalric code changes, hinting to the reader that something new will happen in order to help the author achieve the wanted interpretation of the Arthurian legend: each time the Lady reappears in Le Morte d'Arthur, it is at a pivotal moment of the episode, establishing the importance of her character within Arthurian literature, as she transcends any notoriety attached to her character by aiding Arthur and other knights to succeed in their endeavors, subtly helping sway the court in the right direction.