The History of Galadriel and Celeborn

[elbenprincess]

They tend to be characterised not by not taking part in "action," but by being preposterously good at it.

If that is not Luthien, I don´t know who is Her action was putting Morgoth to sleep and in that she was very good.

but a character the author has become excessively fond of and consequently makes too omnicompetent and perfect

Well Tolkiens wife was an inspiration for Luthien, so I would say that Tolkien was very fond of her and very perfect Luthien was also.