I think an argument could be made for both of these options.
Nya as a Flat Character
Salva is a truly dynamic character, so Nya often can seem flat by comparison. We get to read very little about her, so she also doesn't "get as much time" to show true character development, so to speak. Nya has the same goals and desires throughout the novel (getting water, going to school, seeing her family safe and happy) and achieves them primarily through the actions of others. By the end of the novel, one could argue that Nya's environment has changed, but she herself has not.
Nya as a Dynamic Character
One could also argue that Nya is a dynamic character. Through the novel, she matures as she faces new challenges and opportunities. Most notably, she takes significant responsibility when her little sister becomes ill. We also see Nya growing more worried for her family when the seasons change and they become more vulnerable to tribal violence. Finally, one could argue that we see her become a more hopeful character as she realizes she will no longer have to spend her days gathering water and can instead go to school.