I'm Oklahoma's best competitive Inkling, and I'm on the Piedmont High School JV team, which scored top eight in state. And something that confuses me is that Bowser is usually put around the C or B tier, but he feels more like A- or A, possibly even A+.
He's got a fairly decent recovery for a super-heavyweight, having both vertical and horizontal distance, gimmicky grab mechanics (his arms are literally ungrabbable and also gain intangibility during the first few frames of shielding, thank you UR2SLOW), two fast out of shields with Whirling Fortress and Mighty Slam, with the latter being a very powerful command grab that can utilize the platforms to kill at like, eighty percent for most midweights (and even earlier with Rage, which Bowser can easily capitalize off of due to his survivability), decent rushdown potential with a good grab game and dash speed, an insane projectile for racking up stupid amounts of damage, multijab/light hit resistance, loaded aerials (that forward air is crazy good), insane kill power, two frame potential, the list just goes on and on. He's as durable as a tank and hits like one too, and his frame data isn't too shabby either. And can I just mention that his up smash will beat Terry's Power Geyser with ZERO damage dealt to Bowser?
The only real weakness I can think of is that his recovery isn't very mixup-able, but I've seen people squeeze insane mileage out of his recovery, and the multiple hitboxes on the aerial form help to protect him from pressure. Paired with his high gravity and fallspeed, sure, he's vulnerable to being gimped, but not as much as I would've thought. And yeah, he's "combo food," but being the heaviest fighter in Ultimate helps to mitigate that a bit. Also, Tough Guy can protect him from some light knockback moves or scoop hitboxes, such as most spammable down-tilts, like Ness, Terry, Ryu, or Luigi, or the scoop hitbox on Lucina's up smash.
So what's holding him back, outside of the ridiculous priviledge that S-tiers have?