One piece as a whole is excellent at character developement. All of the crew continue to get stronger but usually in unique ways. Nami and Ussop have to use their brains to outmanuever their foes by using tricks and illusions. Also even at this point Nami has quite a few redeeming qualities. She's intelligent, strategic, and analytical making her a sort of reality check on luffy cause sometimes he can be a little impulsive. Nami is also most likely one of the best Navigator/Meteorologists in the world. Which becomes extremely important on the grand line. At first in the series she isn't much of a frontline fighter because she isn't superhuman like zoro or luffy in her strength so she is forced to use her intelligence to take down foes. Later however she gets an upgrade and is able to fight on the frontline in a unique way. So yes she does get improved throughout the show.