Yeah well...
"Show me your moves!"
Anyway, Bowser. You're all being a little too quick to write Bowser off as a sluggish dope, but in reality, he's got a lot more up his sleeves to call upon.
First, examine his SSBM capabilities. First and foremost, he's a tank. He can take abhorrent amounts of damage and not even feel it. On top of that, he's not much slower than Ganondorf, and he's significantly more powerful. At his disposal he can use fire breath at close-to-medium range, his Claw Swipe allows him to grab his enemies so firmly that they can't esacpe allowing him to dig into them with his teeth, his Bowser Fortress attack makes him imprevious to damage while dealing mass amounts of it in return and his Bowser Slam moves exceptionally quick and is very hard to dodge, especially with a slower character like Ganondorf.
Not only that, But Bowser has a lot of other techniques to call upon as well. In Super Mario RPG he was capable of summoning a Big Boo which would scare all enemies on screen, lowering all of their combat parameters. He could also summon a cloud of poisonous gas which would cause a continuous loss of life to the poisoned enemies. His Crusher attack would cause a giant stone spire to eject from the ground below an opponent, causing a ton of damage. Finally, his Bowser Crush attack would summon a massive Mecha-Koopa to stomp the life out of everything in it's path.
On top of that, Bowser has special abilities and attacks from any of the various Mario bros. games that he can use. He's got that weird clown-copter thing that he can toss Mecha-Koopas from, which would add a ton of mobility to his strategy, making up for how slowly he moves.
And finally, my favorite additional ability, as a last resort Bowser can transform into Giga-Bowser, as is evidenced by Super Smash Bros. Melee. Check him out, he kicks mondo ass.
He's about ten times larger than normal Bowser, he takes about five times as much damage to kill. All of his attacks are the same, only much larger and much more powerful. Heh heh heh.