Toriyama created Lightning for Final Fantasy XIII series. He can name any of his own characters he wanted with myths and different meaning of the characters. Like Cloud, Squall, and other, he can think of something interesting and cool, whatever.
For example, if I create my main character as a heroic demon who is sympathetic to humans and fight the evil for them and name him "Drake" or "Red Scale (code name or nickname) ", it can make everyone remember his name of who he is when they play the game, despite of whether it's good or bad.. --- "Get up!! Show me your true power!" Ryu's victory in SF3
I don't really know how common it is, but my brother-in-law, my best friend's cousin, and two of my other friends all have nicknames they got from the military. And compared to their nicknames (which are not at all silly IMO), Lightning seems like a decent nickname too. --- Final Fantasy Series Live Action Cast: http://www.youtube.com/watch?v=z17xfNHwbBM