I'd be happy with this chart if 3 and 4 got tweaked a little, it is a pretty good way of doing the boss progression besides them.

Playing against Mad Jack, his main moveset is trying to flatten you with his Box form, so you have to jump/hover to platforms to dodge him. When he does stop, there's 2 buttons that'll come up on the platforms at random, that can electrocute Jack if you hit the button that matched the color that Jack's currently on. If you get it wrong, you get zapped. And while you're doing that, Jack throws fireballs, and in his 5th Phase, Lazers that can glass an entire platform and knock you back off. On the 4th, he even turns on sv_cheats_1 and hits the invisibility cheats, but you can still see his particle trails. It does get pretty sadistic, but it was really satisfying completing it. Jack even slows down once he catches up with you or when he's over your platform, so there's some mercy to it. Just wish the game could give some hints on it.

THE PUFFERFISH on the other hand, is anything but fair. You have to use a boat to swerve around a pond and go through rings to electrocute him, which get progressively smaller each phase. The puffer's main attacks are random spurt fireballs. Later on, he also uses Fireball Rain, trying to target the player better with each round, Shockwaves where you have to skim the edges of the pond to dodge, and even Seeking Pufferfish, that'll lock onto your boat and wont get off unless you shake it off like Top Gun. Trust me, it's a lot more difficult when you're playing it (with an n64 controller) because the boat controls are janky. Never anywhere else in the game are these boats used anywhere else other than in this boss, so there's no prep for what to do.

But yea, the other bosses are alright with difficulty scaling. Only gripe about the final boss is that it could use hints (Like the Announcer from Battle For Bikini Bottom) on how to approach certain phases of K. Rool, since each one requires a different setup. They could have made Kranky give advice to the player for these things, because if you run out of time you get to talk with him, but instead Kranky throws his back out trying to demonstrate. But yeah, that's all I got. Thanks for reading my lil rant.