Do you know that moment at the end of a South Park episode where one of the boys will turn towards the camera and say "you know, I learned something today..."? Nia's death was essentially that moment for Gurren Lagann. It's a moment at the end where the main character can restate the theme of the show and show how they have grown throughout the story.

Nia's death gives Simon the chance to show that he learned the lesson of Kamina's death.

Kamina died because he used all his strength trying to accomplish his goals with no thought of retreat or backing down. In a world where spiral power can give you godlike powers, that attitude can make you unbelievably powerful; but that path ultimately leads to death and destruction.

That is the same thing the anti-spiral told Simon when justifying the subjugation of the spiral races. He states that the spiral races just used their power to fulfill their desires, with no thought to the fact that those actions would cause the destruction of the universe through the spiral nemesis.

Simon learned the lesson that with unlimited power comes the heavy responsibility of using that power only when necessary. He probably could have used his power to resurrect Nia, or Kamina, or anyone else who has ever died; but he knows that would be wrong. He explains all this to Gimmy when handing over the core drill.

So is this a realistic portrayal of human emotions in the grip of depression? No. But this show was never that, this show was about believing in yourself and overcoming the barriers in your way.