To be honest, Mario is legit as **** Top 5.
So, to begin with my argument......
Mario is a grappler, arguably one of the best in the game.
He has an extremely large amount of versatile tools, a good combo game, and decent kill power.
In terms of MU, he has a positive MU against most of the top tiers, excluding Cloud, but most notably, Mario is considered by both the Sheik discord, top level Sheiks, and the Mario discord, to beat Sheik, which is an extremely good MU to have.
However: his faults are that any character that plays the range game against Mario can beat him, and due to this fault, he has a large amount of losing MU, however little he loses by, they are still losing MU.
What makes him such a good character is the fact that he has so many tools to use in so many different ways, the players have leeway to have their own individual fighting style, which also means that Mario has the potential to overcome the majority of his issues, however, this is based entirely on the players abilities.
Certainly, there is the argument that all characters depend on the player abilities, but that is both true and not true. What I mean by this, is that certain characters are limited in their options, which leaves little to no room for flexibility, which therefore means that their fighting styles will be limited to what their characters options are supposed to be.
Why this would have an effect on the match is because of what everyone refers to as "the smash 4 factor" which is supposed to refer to rage, but you can also translate it as bull**** jank early kills.
Mario has a very high smash 4 factor, since he has a numerous amount of tools to gimp you at low percents, and rage Mario can
sometimes take you off the top for an early death.
While I do not deny that Mario is a good character, people have a tendency to overrate him, most likely because they either play a character that gets destroyed by Mario, don't know the MU, or simply is just going along with the Ally hype.
Mario is certainly top tier, but he is not top 5, and not top 10, simply because there are at least 10 other characters that can perform better than him, if not only due to his limitations in a lot of MU (there are a lot of sword characters in smash, and they are popular characters)