It's not like
If that were the case, characters like Terry Bogard or Joker wouldn't be in Smash.
Not saying popularity never matters.
Else we wouldn't have Ridley, King K. Rool or Banjo-Kazooie.
However, it's not the most decisive thing, when it comes to making a character for Smash.
Not to be rude, but popular in your eyes or popular from an overall standpoint? Terry has been a playable character in 37 different games, a number I'm not sure that any other character maybe bar Mario has reached. Sure, they're all Fatal Fury / KoF games, but he's playable in 37 of 'em. It's not like he's a side character in a game that no one has ever played before. It's also not saying he's a household name like Pikachu either of course.
And Persona 5 (Royale) is one of the highest rated games on PlayStation 3 and PlayStation 4. It might not have the audience of Final Fantasy as it's more specifically a JRPG of a certain subgenre, but on most lists of best PS3 / PS4 games (especially RPG based ones), it ends up near the top, at least Top 15. For what it's worth in this case though, it's probably more the game as a whole and less Joker, but again it's not like it's a random nobody game.
Characters like Jigglypuff, Ice Climbers, R.O.B and Wii Fit Trainer refute that claim.
At the time she was added to Smash 64, Jigglypuff was popular. (Though not as popular as Charizard, Mewtwo, etc probably) She was probably added since the work already went into Kirby float mechanics. I'm with you on the other 3 though. They're all pretty random, though at least Wii Fii was somewhat relevant a the time they were added.
I think the difference between base roster and DLC though has to be in part popularity based. It's easier to sell a popular character than a less known one. I don't have a great guess as to the final 3 DLC of this pass, but if I were guessing for money I'd be more inclined to pick characters from long running series, popular Game of the Year types games, or a character that has a decent amount of appearances.