I like the 'childish' concept of her personality for two reasons:
1) It would be really boring of I got was characters who were dead serious about everything and didn't have their faults so blatantly visible. Sure, every single character in the game has some fault, but Sera's seems the most 'extensive' just because its more out in the open. Kinda adds to the whole 'faulted hero' concept that NO ONE is perfect. Not even the characters we make or come to know. I mean, if people didn't like her, they don't have to recruit her, and can kick her out later!
2) Sera's childish personality could be alluding the concept that EVERYONE is somehow affected by war/conflict; Including, as much as we hate to think it, children. Children, have no concept of what war is like, its consequences, etc. They have a general idea that its one side versus another, but children don't understand declarations of war for moral reasoning.... Or religious, political, economic etc. Sera, to me, seems to play off on that idea. It's not that she doesn't care about the war, so much as that she doesn't fully understand it; Aside from the Templars v Mages thing. That's not to say, she is stupid, but more of that she is young and 'innocent'. Just because one grows up in a 'bad' environment, doesn't mean they know how the world works. Sera kinda reinforces the idea since she thinks war is simple, in terms that it can be solved by either one side winning, or all sides talking. Unfortunately, this is rarely true, as everyone tends to have their own agenda in warfare. To top it off, I think Sera says that in one conversation, that she wants things to go back to the way they were, in that they are simple. It kinda reminded me of the South Park episode the aired right after 9/11, and how the opening line was, "Do you guys remember when things weren't so complicated?", or something along those lines. So basically, kids don't understand conflict/war, but they see what it can do to their society from watching how everyone, mostly adults, react. They don't understand it, but they know something has changed drastically, and it is affecting them. This then becomes more of an issue of kids trying to maintain a life they are familiar with, rather than changing it because of some war they don't understand.
That's my two cents anyway.