I did not expect to like her as much as I did.
That said- to the question at hand- I think that she's less of a Mary Sue and more of a plot device. From what I know of Mary Sues, they're usually created to fulfill a writer's inner desires, whether or not they have flaws in appearance or personality. I don't think that's the case with Tauriel. I believe she was there to add to the plot in different ways.
For example, adding her character created a romantic subplot, had a female added to an otherwise entirely male cast, and made the elves (particularly the ones in Mirkwood) more sympathetic. I think a big bonus in adding her is that it allows both Thranduil and Legolas to be pretty horrible. It gave Legolas' character in the LOTR movies a little more depth-- not for having feelings for her, but because you can see how far he's come as a person from the snotty princeling to a valuable and loyal member of the Fellowship. And I feel like there is the added implication that he learns something from her, particularly in how he comes to treat Gimli.
I honestly don't think it was a bad decision to include her. I really liked her subplot with Kili, and I didn't hate the thing about Legolas liking her, because it ultimately doesn't seem to lead anywhere that's not in the books. I have significantly more issues with the Hobbit movies, so I was really pleasantly surprised with Tauriel. biggrin