Tiers are only a measure of how intuitive a character is, not of how much better it is. Some characters are simply more approachable than others. That doesn't mean they're better, it only means they flow more naturally. Bowser, for example, requires much more strategy (as the article states) than Sheik, not because Sheik is easier to master, but because she's easier to pick up to more players than Bowser is.
Maybe 1 out of every 200 players feels Bowser is more intuitive than Sheik, but since the other 199 feel otherwise, Sheik will no doubt be much higher tier than Bowser, because she's simply more approachable by more people than Bowser. Thus, tiers are not a measure of who's better than whom: they measure how approachable a character is to how many people.
That said, if we start to take notice of lower tier characters and focus on how to play them at a competitive level, we'll no doubt expand the Smash universe, and create greater variety of gameplay styles. And that can never be bad, now can it?