The roster was finalized in 2015, long before Samus Returns. It had no impact on Ridley getting in. And that's a double standard. Waluigi getting snubbed made the news. Sakurai will likely at least reconsider him.
Pulling out random props isn't a great idea for a moveset unless it revolves around that being the theme, and we have Game and Watch for that. Like I said, a good moveset isn't just a set of moves.
You can't say popularity won't be enough because it was enough for Ridley. Importance doesn't really matter when every truly important Mario character is already in, save for Captain Toad. And even then, Chrom was more important to Awakening than Robin and especially Lucina. Importance isn't the sole factor.
There's no point in considering Waluigi in the first place, though. Importance definitely is an... important, heh, factor. Chrom and Robin were of equal importance because while the Lord is Chrom, Robin is the player character and who the latter half of the story revolves around. And Lucina was an EZ clone so she's a whole different case. Not only that, but Robin offered much more than Chrom did, making Robin the obvious choice to add from Awakening.
We won't agree on the moveset fitting Daisy, so I'll drop that (but "coherent moveset!" isn't an argument for why a character should be in).
Ridley definitely had more going for him than his popularity. He's from a smaller series (he's the third Metroid character), he holds an important role in that series, and he's had major roles in the previous Smash games.
Waluigi
only has his popularity, and when he has to go up against Toad, Paper Mario (who, yes, he is important, he has a whole series of games, doesn't matter if you don't like him), and Geno, even (who Sakurai has personally wanted to include since Brawl), that popularity won't get him anywhere but his Assist position. Which he's been for the past three games (including U).
