Waluigi will never be a playable character in smash, only characters that are very important or playable to/in their series become playable characters in smash. That's why Dixie fits perfectly. Waluigi was only created to be warios partner in spin off games, so he yes he starred in more games than Dixie because we get mario spin offs frequently and because of that and the huge amount of copies spin offs like mario kart sell he is of course better known. But there are also characters like boo, birdo, koopa, shy guy and many others that appear in the same games as waluigi, should they also be playable in smash?
If you asked me six months ago, if I thought Dark Samus would ever make it into Smash Bros., I would say never. If you asked me about Ridley ever happening after Sakurai saying he thought Ridley's proper place in Smash Bros. was as a boss, I would also say he most likely was never going to be made playable.
Look at Geno, he is literally just a one-off character from a game released over 22 years ago, and his only appearance since then was a cameo in a mini-game for a game released in 2003 (and in the remake for said game, his cameo was removed entirely). Yet, Geno is on the cusp of getting in as a newcomer, and is one of the few characters (or perhaps the only) the only to been stated by Sakurai himself that he wants to see him playable. I fully expect Geno to be made playable through Ultimate's DLC that is how confident I am of him getting in.
The fact is I actually see Geno and Waluigi BOTH getting in before the likes of Toad, Paper Mario, and Toadette (who has built up an impressive resume in very recent years to the point she is now referred to as a rising star by Nintendo themselves), even though all three of those characters have more importance in the mainline Mario series (Toad and Toadette) or have their own sub-series dedicated to them (Paper Mario).
Having a large, screaming fanbase goes a VERY long way for getting into Smash Bros. unfortunately (which is sadly also a contributor as to why both Dixie Kong and Toad have not made it in yet).