Honestly, Waluigi is in a really good spot to get in(as is Toad. Captain Toad to a lesser degree). Both are at the height of their popularity and pretty majorly notable characters. Toad is the biggest missing mainline character, and Waluigi is the biggest missing spin-off character(who has made minor appearances in the mainline series now too, via costumes, though. But then again, "everything is canon in Mario" anyway, though there are some alternate universes. Paper Mario was made clear to be a different one from the main one, and you can probably consider that the same otherwise for the CD-i game, or at least Nintendo wants you to think so, despite having no real reason to be. The two movies are probably a case of that too?).
Also, characters aren't "memes" unless they were deliberately made strictly to be one. Waluigi was made to be a character to partner to Wario and to be an evilish counterpart to Luigi, while being based upon **** Dastardly to many degrees. He's a fairly unique character that focuses on tying in with Wario's more pragmatic approach(compared to Mario's heroic way of looking at things. As well as not willing to cheat at a competition). Wario being short and stout even somewhat works well since it calls back to ****'s own dog(I forget the name).
Pepe is a Meme. Waluigi is a character who was popular for his own merits that became so popular that people created memes about him. The first real meme was from Brawl in the Family, a fan comic, which required them to have fun with a fully established character. It's pretty impossible to be just a meme when he has other traits either way, making that statement a giant fallacy and never made any remote sense. And who cares if he's popular as well due to some memes. It's no less legitimate popularity than anything else. Popularity doesn't matter why. It means people like the character. End of story.