Jigglypuff is a viable character now, but she's not as dominant as she used to be. The guy who mained her switched to Fox and Falco after he started being usurped as the best jiggs player. These days he's the definitive number one player, while the usurper is a tentative number five. People are learning how to avoid rest better, so jigglypuff's competitive potential is waning a bit.

I find it fascinating how much the rankings depend on individual innovators. Some characters that have amazing depth never get developed because on the surface they're crap. The best example of this is the Ice Climbers. The tech skill for those characters is so different from all the other characters that they were underrated for a long time, until one or two players figured them out. And they still haven't been perfected. The top ranking IC player today got to that rank without using their best technique: an infinite grab that's inescapable. They still have untapped potential

Maybe a better example of this is Yoshi. The Yoshi meta basically didn't exist until this year when some japanese guy comes out of nowhere and starts taking games off the top 5 players using this character that used to be F tier. Some of that was probably lack of experience against a strange character, but it's still crazy that Yoshi can be viable in that way.