Probably nitpicking here, but quick question: What makes Undertale and Waluigi 'meme picks'? Undertale has always been a rather popular indie title and probably one of the few that could contest Shovel Knight in that regard, so it's not like the popularity of its choices come from nowhere. Similarly, despite being from the spinoff games Waluigi has always had a fanbase as proven by his support thread throughout the years and the fact that fans of his were disappointed when they saw Daisy get in only for him to be excluded again. Neither of those feel like they're joke picks done just for fun like Shrek, there's a legitimate popularity to those choices and I don't think it's fair to write them off as such without good reason.
Both characters definitely have their popularity, that's undeniable.
With them being "meme picks", there's not only the meme culture that surrounds both characters, but:
1) Waluigi gets a reaction of, "He'd be the number one AT upgrade! He'd also be perfect as the last pick as a joke!" Or, "that'd be soooo Waluigi like!' This type of stuff is said all the time, so when a leaker shows up and says, "Waluigi is in", people jump on it and are like, "It's actually happening!", when clearly there's more than enough reason to believe it's not going to happen in this game.
2) Undertale...I guess
does have a notorious meme culture (just think about how much Sans is at the center of memes everywhere), and it's one of the most prominent Indie titles involved in "leaks". There's also the fact that compared to Mr. Indie, Shovel Knight, Undertale is pretty much treated as the GOAT by the fans of the game. Which is fine. But if you ask me, when there's an indie pick, it's going to be Shovel Knight.
Even one of my close friends said it himself. He's a huge Undertale fan, but he himself said Undertale likely wouldn't happen until Shovel Knight got in, popularity be damned. I realize this is anecdotal.
Basically, both characters get a similar reaction, and have memes posted literally everywhere. It's likely a biased opinion, but I frankly see them as meme picks for Smash. It doesn't mean there isn't real, genuine support for them. They deserve the support. I simply don't think they're going to happen, and appear in leaks frequently
because of meme culture.