Personally I prefer a 3rd animal crossing to be 8 characters in 1 they all are not really fighters so they can pretty much do anything
does
A character shouldn't need to be a Number-in-One Package to appear interesting. I know Smash made some of these characters popular alongside fighting games with character costumes, but the thing that makes Smash a big deal with it's characters is how the characters feel true to themselves. How they feel very close to their original identity. Making a moveset for the purpose of fitting as many characters as possible comes across as less trying to stay true to the characters and more trying to stack as much as possible without stability to the characters themselves.
Marth's moveset was made with Marth and Fire Emblem's gameplay in mind, but Lucina, Roy and Chrom were added because they did not feel out of place using Marth's moveset since they don't play so differently from each other in their main games. And while Simon does take some moves from Richter, both characters don't play differently in their own games, with the focus on the whip's range and items.
It's really the Koopalings who feel odd because it feels the reason the Koopalings probably got in was because Bowser Jr. was more so focused on the Clown Car rather than Jr. himself, with all of his attacks and specials being based around the Clown Car and other tools that Bowser and Bowser Jr. used with the Clown Car like hammers, Mechakoopas and whatnot.
And while the Clown Car has become rather synonymous with Bowser Jr. in later games, only his Final Smash as Shadow Mario is the closest thing to the original identity of Bowser Jr. The Koopalings were probably added as a possible bonus that wouldn't be out of place with the Clown Car, since the Koopalings are rather synonymous with the Airships from SMB3 and NSMBW/U, and if Bowser Jr. was based more around his original identity with the Paintbrush, Shadow Mario and whatnot, the Koopalings may not have gotten in at all, with how out of place Shadow Mario already feels for them when they use their Final Smash.