"Maining" is people who like a character so much that they want to play him at every opportunity. If that's how you feel about Lux, that you have so much fun playing with her you want to play her every game, knock yourself out. If not, well whatever.
Not the strongest but so what? Don't worry too much about what some guy said in a viability rating, tier lists, or some awful "Who you should play in solo queue" yt videos and the like. There is a guy who got to masters playing freaking Heimerdinger for crying out loud.