The problem with Mario is that nobody is using him. Sure there's A Rookie, but as amazing as he is, that's just one player, and one play style. There's not many players outside of him to watch and observe, take notes, etc, which severely limits Mario at the moment.
What if he could execute up b walljumps as perfectly as aMSa could throw eggs on the ledge, parry? What if he could shield drop as good as aMSa and Axe, the only two players making people consider Pikachu and Yoshi being above every other mid tier character?
Sadly, until A Rookie becomes Axe or aMSa in terms of technical prowess, or just pushing Mario as best as he can, Mario will remain at the bottom of the mid tiers. I think that outside of the top 8 from the last tier list, Samus, Pikachu, Luigi, Doc, Yoshi, Ganon, and maybe even Young Link could pass Mario. Should Mario be that low? No. He has a lot of untapped potential that nobody has perfected yet, and not enough players are using him seriously. As a Mario main myself, this is sad to see, but it's only inevitable in the current meta.