Well, how about we start with defining what criteria a character has to meet before being placed in a specific tier:
Top: These characters can be untouchable with a good player. They can quickly rebound from damage, have good hitboxes/damage/knockback, and they can camp or approach successfully with relative ease.
High: These characters usually have one minor flaw that keeps them from being at the Top, like Olimar and his easily gimped recovery.
Middle: These characters can do well in tournaments if used by the right people, but still lack the required criteria to be considered especially good or especially bad.
Low: These characters, despite the player's skill level, rarely perform well in tournaments, and lack the neccesary criteria to become even adequate. Maybe the attack hitboxes are small or they have low priority on their moves.
Bottom: Yikes. These characters can either be very VERY hard to master/control, or just hopeless.
Can we agree on this? I might've made some mistakes or forgotten some key details, so if you want, go ahead and reply with your own ideas.