I think the best part is Scorpion was declared in-universe the Hero of Mortal Kombat in one storyline(Mortal Kombat Deception).
I expect him regardless over anyone else(maybe Sub-Zero, another key mascot, is possible) in Mortal Kombat because he's not just the mascot, but he represents the gameplay extremely well too. Mascots don't need to be protagonists either. Just being a main character is also important.
Likewise, Fulgore and Nightmare are the pretty obvious characters too, being not just the mascots, but are very easy to represent the core gameplay. "Protagonist first" can sometimes work, but it's not always cause they're the player character. Villager is the best example of where the player character is the best move possible to represent the core gameplay. Hero's an odd one since the thing that makes it work best is the alts, not simply the moveset, where the mascot could do practically all the same moves(just not without a lot more work due to more animations needed. Capabilities is not remotely the issue between them)>
More or less, it's why I think "Protagonist first" is misleading. It's more "A Main character first", which fits a lot more accurately. Main characters don't have to be heroes or protagonists alone. They can be a villain or an antagonist and still be key to the game they're representing. Pikachu is an unique one in that regard. It's absolutely a main character, but is in no way the protagonist of the series either(besides that, it was more loosely based upon the anime version, where it's more like an extra to Ash, who is the definite protagonist there).
I do think sometimes mascots have an advantage(Scorpion, Nightmare, Fulgore) because that's who the companies want to push and they're the most recognizable. But it's also a matter of context. As I noted, Villager is the one that's the most logical to represent the core gameplay overall. Nook nor Isabelle are strictly about that alone. Nook is actually a lot more about that than Isabelle too.
It's best to say that it's not nearly as strict of a rule and it's a bit more "broad" in some ways. Sometimes the core gameplay is represented strangely, like with Shulk and some inaccurate Monado Arts. Other times they get the core idea down with Hero overall(and technically the equipment factor via alts, something that Slime couldn't actually do in a twist), by showing off multiple games at once.
vs.
