Wow, seriously? Does a show have to have a Disney ending for you to be happy? Did you not see episode 8 or episode 25? While it was certainly chock full of fun times and laughs Gurren Lagann wasn't the kind of show that came with kiddie gloves glued on. One of the central themes of the series was that bad things happen, even if you have the best intentions, but what matters most is that you persevere despite hardships and barriers and try to make the world a better place than it was before. I found that message to be one full of hope not despair and I find the final ep of GL rather uplifting overall.
Sure it's sad that things between Simon and Nia turned out the way they did but did Simon let that depress him? No. He knew that it was the natural consequence of what he did to save the universe and he made his peace with that like a man.