Nia developed plenty through the show, I believe. She had a purpose in the narrative, thematically speaking, even beyond helping Simon to develop. She started as naive, sheltered, so-spoiled-she-doesn't-know-she-is-spoiled princess without much information on the world beyond the absolute reality Genome had provided her with. She starts shaping herself through her interactions with the cast and comes to appreciate Simon because of his love for Kamina and his dedication to any task. To her understanding, Simon's statue of Kamina is the very definition of humanity. Genome's abandonment stripped Nia from her identity, it took away all she believed was correct and Simon appears to give Nia something to believe in, something to validate her own existence.

It just grew from there, really... and that's more than enough to guarantee their relationship as compelling. Besides, let us not forget that maybe Nia saw a little bit of Genome in Simon; you can't afford ignore the possibilities of a father complex when it comes to Gainax shows.

Regardless, perhaps that's not for you and I understand it's just silly to argue about personal preferences; but there is plenty of material to understand the development of their relationship.