By the logic behind Jigglypuff, we'd sooner remove Ness and Captain Falcon from the game, as they're both far less well-known characters outside of Smash. Jigglypuff's been in a game released this year. C. Falcon, not so much, but not forgotten. Ness hasn't been in anything since the SNES, man...
And what about Pit? One game, on the NES, that sold poorly VS Metroid. One GB game that everyone forgets about. Less of a priority than Jigglypuff popularity-wise.
Jiggs isn't a huge character amongst the "core" smash audience, but for the Pokemon crowd, leaving her out would be a devastating blow akin to leaving out Mewtwo. She's an essential representative of the franchise in Smash Bros.
I mean, would you want them to take Dan out of Street Fighter just because he wears pink and is a silly character? No way...