Yeah, whenever there's a fighting game, people will make tiers with it. Usually to depict a character as great to use to sucks.
Coincidentally, Fox is in the top tier, along with Marth, in Melee, although they both continue to move up and down in Brawl Tiers.
I don't understand tiers either. From what I can tell, its just who is better then who. The problem is, the list is always changing, especially if someone wins a professional tournament with a "bad" character.
My main enemies in Brawl, and Melee as well, are the people who go on and on about Tiers, mostly because I prove them wrong often. I can win with Mewtwo, Bowser, Pichu, Ganondorf, and many other "Low Tier" characters in both games.