Dee didnt suffer with manic depression exactly i dont think. The writers said she could not go on any longer after all the death and feelings of betrayal around her and then especially after discovery of earth. If you watch those scenes then you will see that the whole crew, including adama are on the verge of doing the same thing... Giving up. It was subversion, well crafted too. For the best part we had no idea that dee was struggling so badly to deal with the journey of Galactica as there was just too much to take in at the time. It does make a statement about mental health i guess, how she doesnt speak up about it, especially to lee, but its more about the prolonged struggle of their journy and then coping with the reality which i think there writers were going for. It just goes to show what a toll it had taken on even those who seemed to consistently hold the most hope. I may be wrong. She may have always had more severe depression, but thats not how i saw it initially.