Marth and Lucas were Japan exclusive before Smash.
While this is true, its not a very good comparison.
Here are the problems with arguing Marth and Lucas:
1.) Fire Emblem was still a fairly big IP in Japan, and they weren't planning on bringing Marth over. They eventually reluctantly brought him over. It also had 5 games already, with a 6th released just after Melee.
2.) Earthbound had already been localized. Also, Lucas was planned to replace Ness in Melee, and Earthbound 64 was supposed to be a worldwide release before it got cancelled.
While Panel de Pon has always been a worldwide released IP, the characters have been replaced every time in localization, its a much smaller IP than FE, and because the characters never once got localized, that hurt Lip as well.
If Panel de Pon had been localized with Lip to begin with, she likely would've been playable in Smash by now given that its a longer running IP than Mother and has had consistent western localizations. But bad circumstances have consistently prevented that from happening because its constantly getting localized with characters from other IP's.
Hopefully this Lip costume being worldwide says at least a little something about a shift in perspective. But I can't help but recall what Joshy said when I randomly decided to watch his video on the Animal Crossing direct:
"Hey, that's the girl from Captain Rainbow!"
Granted, that's how I knew it was Lip. That was during the AC Puzzle League segment and it showed a villager model, but I didn't realize she was dressed as Lip (I didn't pay that close attention) until he said that and I was like "wait, wtf?!" and went back to check.