Reality is, any content from a game counts as representation of that game.
Reality is, Smash was designed where the characters were important. This is how it came to be. It was going to have generic characters, but it was decided upon that having recognizable playable characters is going to make the first game sell better. So it is true that playable characters play a major role in representation. It's the game's premise.
It's just ridiculous to think that you need tons of characters to properly represent it too. Every franchise is different. Some don't have easy item options(like Fire Emblem). Some were lucky to have cool moveset ideas but also were easy as clones. Not every franchise has that option overall. I honestly feel Fire Emblem is represented well because the focus of the series in itself is about the characters. Pokemon is focused on the monsters, and both the items and playable characters represent that well. DKC isn't focused strictly on characters either, it has some fun ones that work. It's a mix of what's important. Collectathons, music, unique mammals, etc. Smash actually does a pretty decent job of showing tons of DKC(and respectively the original DK arcade game). What it does lack at best is non-jungle stages. That's the only real thing missing. FE is only missing at best a reference to the weapon triangle, but otherwise does a good job in showing off the series. Super Mario is one of the best represented ones overall. Sonic kind of really needs more stuff from the Modern era, but otherwise is fairly good.
Are characters most important? Only because that's the premise, yes. The rest still matter heavily. Donkey Kong County isn't just about DK, Diddy, and K. Rool. There's so much more to it than that. It's why we get tons of content outside of just playable to help show off all parts of a game. Also, to be frank, I'd say say 2 characters and 30 songs can absolutely be great representation too(Street Fighter is something similar to that). It's all it needs. It's not like it had feasible time to make anyone other than an Echo at best. So it was either the original echo, Ken, or something like Dan, who is nowhere near as popular, or another shotoclone that is extremely similar in both animations and body shape. Which doesn't leave much, since many of them only have a modified moveset based upon the fighting style. You could go on for multiple franchises.