A tier list is a reflection of a characters potential witnessed and recorded via tournament events, with some influence by opinions of how good or bad a character -could- be by established players. If the character doesn't get enough representation, then that character can't establish itself in the tier list properly where it theoretically should be. I'm not going to say that say, Pikachu is a really amazing character in Melee, but if we had more players like Axe who represented the character the way he did, we'd have a clearer depiction of what Pikachu can and can't do, and how he stacks against the rest of the cast.
Most people don't take that in to account when they look at tiers. They just see a defined list. Fact is that the only characters you can accurately assess are the ones at the top because they're winning the most and have the most rep.
Tiers exist, but I think people overestimate their validity in a tournament scene. You're much more likely to win playing a character you enjoy and play well with with your particular style. It's one of the benefits of having multiple characters in a roster rather than just reskins of the same character (Street Fighter 1).