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.
I definitely agree that mascots frequently can and do change, I just don't think that's happening with ARMS. Usually, when a mascot change happens, there's multiple pieces of media that signify this change is occuring. Using your previous examples, Pikachu was the face of a full anime series, virtually every spin-off put out, and an updated rerelease of the original game around the time he solidified himself as the mascot. Isabelle, likewise, received a big merchandise push, became the face of every AC spinoff, and appeared in a ton of crossovers when she was becoming the face of that franchise. Immediately, I don't know a ton of Mortal Kombat so I can't comment on Scorpion, but I think the general point still stands. Mascot changes generally involve the floodgates opening on material with the character.
Min Min has two things going for her: her popularity and her Smash appearance. There's are two big achievements that guarantee she'll be important to the brand going forward, but I don't think it guarantees that she is (going to become) the new mascot. While popularity does occasionally impact who the mascot becomes, there's a ton more cases where that doesn't become the case. There were a good 20-30 years where Donald Duck was far more popular than Mickey Mouse, but Disney never adopted the pantless sailor as a mascot nor did they renamed their animated catalogue to "Donald Duck and Friends".
Her crossing over into Smash is definitely a more salient piece of evidence, given that the characters who make crossover appearances in fighting games usually represent their brand in a style similar to a mascot. However, this isn't a hard rule and there are times where the mascot for a particular brand is passed over for another fighter. Zero and Tron Bonne beat any form of Mega Man into Marvel vs. Capcom 3, but no one would call them the mascot of the Mega Man brand. Likewise, Akuma and Geese beat Ryu and Terry to joining Tekken 7. Even in Smash, Chrom continued to be Awakening's poster boy even after Robin and Lucina were playable in Smash for Wii U/3DS. Min Min's Smash appearance serves as a good start to make a case that Min Min is the new face of ARMS, but additional evidence is needed to prove she doesn't just fall into the latter camp.
Could Min Min become the mascot of an ARMS 2? Theoretically, yes. However, the little winks and nods to Spring Man really makes me think that they're not done with him being the series' mascot. He was one of the two characters involved in the revival Party Crash, his gameplay was tied with Ribbon Girl's for taking up the greatest amount of the screen when it was announced that revealed the ARMS rep, he's front and centre on Min Min's Final Smash, he was the first one to grab the Smash invitation, and Sakurai even took time out of the direct to discuss many were likely expecting him to be the ARMS fighter. Hell, even him pouting in the window in Min Min's Smash Art indicates further attempts on Nintendo's part to tie the brand to his presence.
That being said, does that mean that Min Min's a one-off with no objective merits to her inclusion? Of course not, she's a very cool character. Likewise, Spring Man being the mascot of the series doesn't make him an objectively better character than her, just like her Smash inclusion doesn't mean she's an objectively better character than him. Min Min will likely be featured on the box for ARMS 2 and be one of the series' most frequent characters going forward. Hell, I could even see her taking over the role of ARMS' female mascot from the suspiciously absent Ribbon Girl. I just think it's somewhat premature to start branding Min Min as ARMS' new primary mascot when it seems that Spring Man is also going to still be just as relevant in the future.