That's right, a character isn't always terrible when you use it well.
HOWEVER! Jigglypuff, as a whole, is a bad character due to her very little priority, low weight, and low range. This gives her mostly bad matchups against many other characters, especially high-ranking characters.
In short, Jigglypuff is actually terrible. That doesn't mean every Jiggly player sucks though, I've seen some amazing Jiggly players on youtube.
... Speaking of that, anyone remember Mango from melee? Jigglygod, lol. He's beaten all the high ranking melee-ers as of late.