A Tier List Atm isn't much more than a popularity poll. Many characters are simply easier to master than others and require much less in-depth practice. MK for example has blantatly obvious strengths that are discovered in a relatively short amount of time. People skim over characters and it's a first impression type thing. MK gives the best first impression to players. It's why he's so popular. It's why he gets top-tier. Not because he is broken.
However, other characters are like hidden gems, who need serious studying to be competetive. Don't get me wrong though, I do believe in bad match-ups and some characters may genuinely be better overall, but balance gaps between characters are generally over-rated imo. By now, I'm suprised many haven't caught on to how the characters were balanced to begin with.
I hate to say it but Sakurai partially based the character balance on Final Smashes and items. For example, Sonic has mediocre b-specials that don't show variety. Low priority. Lack-luster aerials. Low atk power and he is relatively light-weight. However, running speed allows him to get to items quickly and he has the best FS in the game.
Meta-Knight on the other hand gets one of the weakest Final Smashes in the game. Hmmmn, I wonder why?