Smiling Lightning...

[Wolf Kanno]

I don't think, it has something to do with her character. Squall isn't smiling the whole game, either. But in the ending, he does. And the first time I saw him smiling, it also looked a bit weird.
But Lightning... she's not looking weird or something, it could be just a cool smile ^^

I will now always think of this when the subject of Squall smiling comes into play...