I found why Mario is High 1-A:
"High Outerverse level (Exists on the same level as the player and
views all of the Marioverse as fiction, the Player is treated as a
great being who watches and
cannot be comprehended, being
treated as a god who summoned the four heroes of light as a gift, would have been completely unaffected by the Void's destruction, which was expanding at a rate that would have allowed it to go from enveloping Mario's 5/11 dimensional universes to consuming realms transcendant of Mario's space and time like the Overthere and platonic ideals in an unspecified but persumably quick timeframe, the Void was expanding at a rate where it went from destroying High 2-A/High 1-C universes to >baseline 1-A structures, with its expansion even being accelerated by the Chaos Heart, and it was never stated or even remotely implied that Dimentio would have halted or even slowed the Void's expansion after he achieved his goals [And as demonstrated by Dimension D, Dimentio is capable of creating worlds immune to the Void even in base, thus he'd have no reason to impede the Void's growth], yet the Player is never threatened by the Void in any capacity, implying immense superiority over even the furthest reaches of the Void's possible growth)"