Is Min Min the protagonist of ARMS? Yes, by the metric that everyone in ARMS is considered a protagonist by the dev team. But the mascot? That seems a little far fetched. Outside of Smash, Min Min is very rarely the face of anything related to ARMS. It's Spring Man who adorns most of the game's marketing, promotional artwork, and even the cover. Even in the Smash content we're getting in this DLC pack, Spring Man was the NPC who took up the most screen time in the trailer, the one who's stage got promoted, and the character who's centre stage in the final smash.
Pikachu wasnt on the cover until 4 mainline games into Pokemon.
Isabelle didn't exist until 2014.
Scorpion took a couple of games to start getting onto the cover as well.
Character based games have mascots develop over time frequently, these three above being some prime examples, and ARMS was always, very specifically, a character based game, where each one was themed, and even individually revealed and promoted before the game even released- the DLC characters got that treatment as well.
Spring Man and Ribbon Girl were the marketing options for the cover. The cover has to explain the game enough to parents who will think "oh, that's cool, here is money so my child can have this." Blue Boy (with a very simple name to explain the concept of the game). Pink Girl. It doesn't make them more prominent.
And now Nintendo confirmed that last part about them not being more important, as stated by ... was it the games creator? Someone high up. I cannot recall who at this moment.
Either way-let's see what happens with ARMS 2. I'd be quite shocked to see Spring Man on the cover.
Hell- it doesnt even matter, because if he was in any way intended to be the protagonist, fans voted on Min Min, the creator voted on Min Min, and now Smash is going to define the characters and how ARMS is seen.