Smashboards

In Waluigi's case, I think when people say that he's a "meme pick", they just mean that he's a character who really only has popularity on his side and not much else. He is a reoccuring character, but only in Mario spinoff titles and he's not really anymore "important" than anyone else playable in them. Daisy is the closest thing there is to a Waluigi on Smash's roster, a highly popular but overall throwaway tertiary Mario spinoff character (though Daisy did technically appear in what Nintendo considers to be mainline platformers, those being Mario Land and Mario Run...if that matters at all)...and she really only got in as a very simple echo of Peach with literally no gameplay differences whatsoever, essentially existing as a glorified set of extra alt costumes/colors for the latter. Waluigi however kinda has to actually be a unique fighter considering how different his body/skeleton is from Luigi or Wario. And there's a question of how much a character like him really warrents that dev time and resources, despite his popularity.

I guess there's also Piranha Plant, who actually IS a unique fighter...but I feel like that's just because Sakurai always likes to include some wacky "surprise fighter" every game, like Mr. Game & Watch, R.O.B. and Duck Hunt before it.