He can swim in the air, grow vines, and has dancing moves. He could probably use a tennis racket in a special move. And he has really long limbs, though I guess he has nothing on Min Min. It can't be "Waluigi has no distinct powers".Anyhow, I think some of the feeling in favor of Waluigi comes from the idea that Smash Bros should "complete the set". Waluigi is a major Mario character, pretty much every Mario character more important than he is is already a playable character, therefore Waluigi should be playable. This makes sense, it lets us have more of the dream matchups with characters of the same franchise and more decisively pit different franchises against each other because each individual franchise gets to have its full breadth of options. Plus, there's the benefit of following/creating patterns in the addition of playable characters, and humans love those. I did vote for Bandana Waddle Dee in the Smash Bros. Ballot, a character whose primary claim to fame is being Player 4, after all. (In Mario terms, Bandana Waddle Dee is the Blue Toad equivalent.) On the other hand, I think a crossover fighter is actually served better by breadth over depth in terms of the playable character roster. Having more franchises means there are more aesthetics and tones the game gets to explore and more locations to visit. They make the crossover more grand and special. I think being able to put Luigi in Dracula's Castle is more interesting than getting to have Captain Toad fight Bowser. If I want a comphrensive Mario roster, aren't I better off playing a Mario video game?.
Uh, I guess I still have to respond to the prompt? Well, given this I would be content with the non Super Mario groups here having just three characters because I would happier with having room for adding other franchises. So yay Yoshi, yay Wario, nay Donkey Kong, and nay Mario? Lets go with that. Yoshi and Wario could use at least one friend to pair up with, though Yoshi does easily fit with the Mario cast.
Yoshi's pretty hard pressed for playable recruits though, considering there is has almost nothing to work with aside from the dinosaur. Poochy is too defined by being invincible, and half the fun of playing with him is in indirectly managing his movements. He would be better as an Assist Trophy or stage element. Baby Bowser's proportions are different from Bowser in ways that make cloning more difficult. Even taking that route, you probably have to replace Flying Slam, side smash dropkick, and down throw flop with different things: Baby Bowser is too small. (You might get away with keeping forward throw head swing.) We really shouldn't even consider putting Baby Bowser in a Koopa Clown Car as a Bowser Jr. clone. Baby Mario is mostly an accessory in Yoshi games, a Baby Mario Smash Bros. moveset would probably fail to feel like the character as depicted in a Yoshi game. So, in my opinion that leaves only two potiential playable characters: Kamek and Burt the Bashful? Ralphael the Raven is non-viable: his memorable elements come from his arena and would fail to translate to playability. I'd consider going with Bandit, Ukiki, Goonie, or Tap-Tap here. So it does make sense that there has only ever been one Yoshi characer even five rosters into this series.