While Monokuma is relatively more interesting to look at, he doesn't represent the gameplay of Danganronpa or what the player does in it very well. Again, the Slime vs. Hero comparison.
While Makoto and Hajime are just regular high school students, Danganronpa's general aesthetic of "shooting" lies, rhythmatically pressuring convicts, or literally cutting down opponents arguments is enough to give them a stylistic and visual flare. Sell Danganronpa with its own style and premise rather than that bear who gets on your nerves a quarter into the game.
Eh, not really true on Slime. It can do pretty much all the spells Hero can do. As well as various Cut attacks(I haven't seen if it has unique animations, so using a slimy tentacle may or may not exist in Dragon Quest Monsters Joker, though).
Really, the big thing Hero had was an easier to work with bodyshape. That, and Sakurai wanted the protagonist specifically. It really wasn't a moveset problem, though, not in the same way. No matter which you choose, you can easily get tons of proper representation of the various magical abilities in Dragon Quest without fail. The one thing Hero doesn't really do well is represent the unique armors and weapons available instead. Just the magic is pretty much it with some very basic(but expected) swordplay(as the sword is often a common notable weapon for the protagonist of DQ. Not in every game, but in a lot, including the original).
Both frankly worked amazingly well to represent the magic of DQ. So it wouldn't matter which one was chosen if that's the key goal. Of course, the goal wasn't just magic, either. More the gameplay in general, but I go back to how the weapons and armor frankly aren't represented at all. Which is sad, since that was one of the best parts about how DQ was, especially the first game, with some really cool armor and weapons that weren't just "damage boosts" alone in every case. While the swords weren't too special beyond shape, the armors had unique abilities that would've been interesting to have. Though hey, critical hits are nice. Hero brought other stuff like various other Heroes via the Final Smash and very unique alts. Now that's something Slime wouldn't have(though the lack of alts isn't an issue either. There's a ton of neat variations to go with. Patterns or colors alone).
