I agree that movesets are not an issue, especially when Sakurai always like to go for the protagonists of series. The protagonist represents the game, which is why we saw Hero over Slime. That was my point.
That said, we don't know if that'll apply to every series either. For certain types of games, I'd think a character who could represent the core gameplay, even if not the player character, still can work. In the case of Slime, that doesn't. Slime isn't the PC in most games, and when he is, it's a completely different genre. Even when he's a main character(Dragon Quest Monsters 1 and 2, where he's your partner. Basically the literal Pikachu in the example), he doesn't have the same abilities beyond spells and just generally able to attack. Also, going to what I said earlier about animations, it actually makes a lot of sense for Hero to be chosen due to having more easy to imagine moves for the non-spells. For Slime to use stuff like Cut, you either would have to give a slimy tentacle or some kind of shapeshifting(including Slime Knight). The issue is that's a lot of extra work for animations. Hero borrows animations from other characters, so even if it wasn't simply because it was the core protagonist of the series in general, animations absolutely make Hero a far more feasible choice to begin with. Sure, I prefer Slime, but development-wise, Hero is the logical choice.
I don't know enough of Dragonropa(I probably spelled that wrong too) to say much. I don't know if Junko would be an ideal choice. I imagine the issue isn't how the moveset works, if she's no different from the player character, though. That said, if the case is more based around that, I could see something like a Hero scenario, with her as the face and using other characters(but I'm guessing the models aren't similar enough to be alts anyway, so that idea probably won't work? Bare with me here as I really don't know the characters. I've seen only Junko and Monakuma as is. I've heard of the series, but never knew it was a Murder Mystery-style Visual Novel. Might not be a proper description, to be fair, though. I'm just going off of what was said here. So I apologize if that's a misleading description).
Though it's true it often applies. In fact, this can also be seen with Animal Crossing, which is why Tom Nook wasn't chosen. He wanted the player character instead to represent the core gameplay. That's something an NPC isn't going to do. A mascot wasn't feasible for the purpose as is. Though it does help that very often a game's mascot happens to be the player character(Sonic, Pac-Man, MegaMan, Ryu, to name a few).
In some cases, that's not workable(though really, the only case is Pikachu at best, as Trainer was nowhere near feasible anyway. Though to be fair, we don't know if he even had an idea for Trainer during Smash 64. I think it was a rumor he thought about it during Melee development).