Unlike many fans of Tolkien I had no problem with Tauriel as a new character developed for the Hobbit films. My opinion of Jacksons' achievement with her is in general much higher than my opinion of his success overall. It was a series of films with moments I loved -- the unexpected party and riddles in the dark scenes in particular -- and one performance I thought was splendid -- Martin Freeman as Bilbo -- but which I thought wasted the assembled talent and the possibilities in the story. Stephen Fry, a longtime favorite of mine, was dreadfully disappointing, and the scene with the goblins beneath the Misty Mountains was as ridiculous as the scene in the mines in Indiana Jones and the Temple of Doom. In fact my reaction to these three bloated movies is aptly summed up in the words of Dáin Ironfoot, as portrayed by Billy Connolly. When the were-worms burst out of the earth before the climactic battle, he cries out in exasperation: 'Oh, come on.'
From the start, however, Tauriel seemed competent, smart, and tough in herself regardless of others. Even in that cringing scene in which the contents of Kili's trousers (now there's a word Tolkien would have chosen to use) are the subject, if not the object, of banter, the goal seemed to be to portray that kind of cagey toughness and wit which Lauren Bacall played so well in To Have and Have Not. Sexy, poised, unafraid.
But she is more than that, too. She looks beyond the borders of her land to see the troubles of the larger world, just as the real Galadriel did before she diminished and went into New Zealand* and turned blue. Tauriel grasps things that the inward looking males of Mirkwood do not wish to acknowledge. She see that her people may be able to fence themselves in, but cannot forever fence the world out. Like Éowyn she fights well and bravely and fearlessly, but unlike her she already knows how to heal. She understands her own heart.
Even so, despite Jackson's success with her character per se, her story went off the rails like an express train. The idea that she had to have some kind of love interest was a flaw from the beginning. It profoundly, almost contemptuously, underestimated the intelligence of the fans in general and women in particular. Still, an underplayed ending to her story might have approached success (faint praise, I know). Yet her preposterous exchange with Thranduil over Kili's body --- 'Why does it hurt so much?' 'Because it was real' -- exploded the credibility of her character, sacrificing all the good of it for one of the clumsiest weepy endings I can recall seeing. One need only compare Bilbo's scene with Thorin, and the silent witness he bears to the toll of the battle, to know how much better it could have been handled. But Bilbo's response to the losses around him has its origin in the pen of Tolkien, who could write and knew death on the battlefield too well.
So the problem with Tauriel is that creating a good character is not the same as writing a good story for her. Consider how much more firmly the makers of Wonder Woman reined in this aspect of her story, so that there even the banter about the needfulness of men to women subserves the larger tale of Diana's realization of her own heroic stature in a world both larger and lesser than her home, a world which the women of Themiscyra have long fenced out. Had Jackson resisted the temptation to follow the traditional cinematic playbook that requires female characters to have a love interest -- had he even asked himself what Tolkien would have done with her -- Tauriel's story might have proved meaningful instead of maudlin.
*No offense to the good folk of New Zealand, which seems a wonderful place. I very nearly moved there long ago.