Neko has a point really. No matter what, tiers are basically a general and simplified matchup analysis. To say it in simpler terms, it's the result of an aggregation of matchup analyses.
Since it's comparative, it's not as simple as looking at one character and deciding that he's good based on his stats and moveset. Falco who was a Melee top tier for example, could theoretically be exactly the same as Melee and then be dethroned from top tier if more characters were made potent against him for example.He would not have changed, but his matchups would. Heck, I think it's possible that a melee top tier will actually be demoted to high tier when the metagame evolves further. You never know.
By keeping in mind that how good a character is is a comparative exercise, there is bound to be an average, and anything near that average will be classified as mid tier. Characters going above that average will be the high tiers and above, and characters going below that average will be low tier and below. It's bound to happen because every character is different. Now, the question isn't whether there are going to be tiers or not, because there will, but whether the difference between the viability between the characters will be a large one or not.
If bottom tier characters have a fairly believable chance at winning a major tournament, then the game is extremely well-balanced.