He's a big goofball when he's not being a royal pain to the surrounding nations.
His crush on Peach didn't start developing until a couple kidnappings later.
He lets Mario win. Why else would he put all the axes, buttons, bombs, Rocket Nozzle platforms, etc in his battle arenas? Heck, speaking of the Rocket Nozzle, he's the one who put FLUDD at Delfino Airstrip.
Kidnappings and fighting Mario started being fun for him and his troops, so now they're also a recreational activity and training regimen. He supplies his minions with infinite 1-Up Mushrooms so they can have fun without worry.
The Koopalings and Bowser Jr are all his biological children *dagnabbit*. I'm the world's biggest sucker for sibling teams, and a team of 8? I'll eat that up!
His shell doesn't come off, same with all others in his Koopa subspecies. His shell's spikes automatically retract