Yabuki chose Min Min. We know that for a fact. Stop stretching the truth here. He requested it to Nintendo, the actual reason Sakurai would be able to even use the character. Nintendo hard chooses the characters, which means any requests must go through them first.
Sakurai in no way chose Min Min on his own, nor requested it. That doesn't fit any of the information we have.
She happens to also be extremely popular(winning the ARMS contest, which isn't really a true tournament but a pure popularity poll), but whether that's why Yabuki requested her to Nintendo is unknown. Spring Man is also Yabuki's favorite, as he favored both characters, being the Mascot of ARMS(Spring Man), and the one he personally wanted for Smash(Min Min).
In addition, Sakurai gave nobody but Spring Man anything special in base. It's very clear he was the only choice to represent the core game during the base game. He didn't have that option for the Fighter's Pass, because he wasn't choosing a character. Nintendo was in all 11 cases(or in this case, 12 cases since one's a two-in-one Pack). Min Min being in was out of his direct control. While his favorites are Min Min and Ninjara, as admitted by himself, Min Min isn't the AT, now is she? No. It was the mascot that got first dibs for the biggest role. It was obvious if time went better, Spring Man was very clearly the de facto choice. It's not till years later than Min Min gained serious enough traction to be considered viable for DLC. She still had to fight Spring Man for that spot among the fanbase, and even they recognized Spring Man as possible. It makes sense. They're the two biggest characters in the game for different reasons.
It's sometimes just as simple as it sounds when it comes to character requests. Don't forget that both Snake and Sonic were originally requested to be put in Melee(this was well before the Sonic series was hitting the GameCube, as that didn't happen for a while. Sega wasn't out of the console business either). Time constraints said "nope", but what'd you expect with the game being rushed out overly fast. Like, the only thing that wasn't originally intended they caught was Wavedashing, and kept it in cause it was a fun mechanic in practice. Snake got requested for Brawl in the same fashion. Sonic on the other hand was a case of, by that time, Sakurai requesting to use him(and he was lucky to make it in, heh).
Character Additions are complicated in that regard too. They go a ton of ways. Sometimes the mascot isn't the first character, but if it's not that, it'll at least be a protagonist(Min Min is one such example), which is a fan rule based more upon logic than just "they had a role we don't think would get upgraded". It makes sense a major character(usually a protagonist, though so far it applies right now) or mascot will get first dibs. Smash also doesn't go out of its way to give first characters necessarily immunity to cuts. Ness was almost cut twice, after all. However, it's now established his role as the posterboy for Mother, with only Lucas ever seeing a cut. It's easy to assume the rest of the series will never have their first character cut unless it's the entire series cut(poor Metal Gear, Ice Climbers). Even a notable Protagonist wasn't immune to this, being Pokemon Trainer, who got cut for technical issues(and they didn't have Squirtle or Ivysaur at least return separately from Charizard. But it does make sense if only one was chosen, it'd be Chaziard. It towers above the other two is popularity, after all).
