The City of Ember is built on hope—no matter how dismal it seems in Lina and Doon's time. Obviously it took a lot of planning to make the dream of Ember (a refuge for humans in a world careening toward disaster) a reality. The Builders were some pretty ambitious and hard-working folks to make it come together. And if you fast forward to the present, we see Lina and Doon with hopes and plans of their own. Not everything they hope for comes to pass, but hey, we're gonna say that hoping in general is better than not hoping at all.
The mayor: too much planning, too little hope.
Ember is a place of unfulfilled dreams, but once they get above ground, they'll realize that that's not the solution to their problems, either.