I hate to be the bearer of bad news but:
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Keep in mind there are more ways to represent a franchise than just playable representation. Balloon Fight, Pilotwings, and Nintendogs all have stages based on them, but no playable character to go along with them. Even simply receiving a stage would boost a lot of exposure for the original cast and setting for Panel de Pon, not to mention all of the excellent music tracks that we would receive. Assist Trophy is another way for Lip and the original cast for Panel de Pon to gain exposure. Even simply additional music tracks are a good thing.
Even still, Sakurai has been known to throw his own rules or opinions out the window when the situation arises. He said third-party characters needed a proper appearance on Nintendo consoles to make a guest appearance in Smash Bros. and yet we got Cloud, he said the Villager would not work as a playable character because Animal Crossing is a pacifist series yet he was the first character announced for Smash 4, and finally he said Ridley was too big for Smash Bros. and being playable was not a suitable role for him, only to do a 180 a year later.
That being said, I do not think Lip is happening as a playable character on the base roster (of which per Sakurai there are only a few fighters left) or as DLC (of which there still would be very fierce competition). There are still things for Panel de Pon fans to look forward to with Ultimate, namely what additional content the series will receive in comparison to Smash 4. Smash 4 was the first game in the series where the Panel de Pon series did not gain any additional content (we even lost some with the loss of stickers). So its going to be interesting to see if the series receives any new remixes, or if there is an Assist Trophy of Lip or any other Panel de Pon character, or possibly even a stage. There are still possibilities to be interested in and look forward to, even without decent prospects for Lip as a character.