Really? Compared to most of the cast, Nia had a very compelling back-story and strong emotional development.
Like Kamina, Nia was a character tied by the chains of her origin. Her relationship with Genome was not indented to give her any kind of "plot" relevance. It was a mimic of Kamina's infatuation with his father. Like Kamina's arc, in which Kamina faces against that same human fragility that killed Joe, Nia's arc ended with her confrontation with her father. Ultimately, she says farewell to his influence and decided to look towards tomorrow. In the end, just like Rossiu's village ends up coming to claim his soul, Nia's origin(Genome's sin of surrender against the Anti-Spiral) came to claim hers.
Her personality was about "questioning everything", what everyone else considered an absolute she tried to actually understand. "Why are you fighting?", "Why do you love me?" or the like. Beyond that, she had core motivations and her personal reasoning on the world. Actions like when she told Simon to use her hands as cushions for piloting Lagann or lines like "Farewell, Father. I'll look towards tomorrow" or "As a matter of fact, we're completely different persons. But I believe that thanks to these differences we will be able to live together" among others are clearly part of her characterization.
You can dislike her, I don't care about that. But your dislike is forcing the writing into a mold it was never intended to fit.