It absolutely does not have to be Spring Man considering he’s barely a mascot to begin with and his greatest accomplishment as a “protagonist” is being on the box getting punched by Ribbon Girl. Like this isn’t comparable to Ryu and Terry who had 30 years of being a mascot/protagonist. This is one character that appeared on a box and has spent 3 years getting BTFO’d by every other character in ARMS in popularity contests.
I mean... ARMS came out 3 years ago. It isn't really fair to compare Spring Man with the likes of Terry and Ryu.
And as others have said, Spring Man is actually popular. I don't know why some people tends to think that all the main characters in fighting games are super hated by the fans or something. No, usually the protagonists are quite popular, and Spring Man isn't the exception.
Not being the most popular character =/= being the less popular character in the roster
Another reason its gonna be Spring Man is because if it came to popularity then Nakoruru would had been picked instead of Terry.
Guess who was chosen? Obviously the mascot and protagonist.
Most of people wouldn't have thought of Nakoruru back then if it wasn't by that infamous 4chan leak about a female fighter with projectiles. If that post never existed Nakoruru would have been discussed after the SNK leak at some point, but Terry and Kyo would have been the most prominent names in discussion. In fact, that was pretty much what happened lol
Nakoruru is one of the lead characters of Samurai Shodown, and she's really popular overall if we talk about SNK characters, but knowing how Smash has been when it comes to pick protagonists/mascots, the most logical choice was always Terry if they wanted to rep SNK as a whole.
Also I never get why people likes to downplay protagonists, according to some Terry or Ryu are bland and boring? Usually said from people who don't know much about the characters or the games, but ironically it's not like Nakoruru or Chun-Li aren't really different. When you're at the point of not knowing really well the characters what is left is their designs and not much more, so it's just a matter of preferences if someone thinks that Nakoruru has a more interesting design than Terry's.
I don't know, if these characters are really bland and boring then for me they're as bland and boring as Mario, Link or Samus.