I haven't really tried her at all, but she just -looks- a little weak.
Does she play like Kirby? I play a bit of him so I might be able to adapt that.
argh I just typed up a long response and the forum did some maintenance so here's tl;dr version:
This is coming from someone who mained kirby and 2nd Jiggz in Brawl...
At a glance, Kirby and Jiggz both have similar jumps and, a few similar moves but they don't play the same. It should be easy to learn one if you know the other though. Jigglypuff can be played more aggressively if needed than Kirby, and also has a much better air and approach game with far superior mobility. She is slower on the ground though, so because of this you will be spending more time in the air than playing a ground game than Kirby, but Jigglypuff received some buffs to her ground game, while Kirby got some nerfs....
IMHO, Kirby now feels out classed by Jigglypuff. He got some really unfortunate nerfs such as combo follow ups on grabs, nerfs on the range of b-air, f-tilt, etc. He already has a poor approach game so now he'll have to work even harder to build damage at lower %s. At least u-tilt is still good.
Jigglypuff can actually approach, has much better air game than Kirby and now can kill easily with it. She also has a decent ground game this time around- they gave her frame data and hitboxes a facelift- notably jab, u-tilt, f-tilt even d-tilt come out much faster.
They gave kirby a better drill and u-smash but the combo and range nerf is pretty bad, he really didn't deserve those nerfs. I would recommend using Puff for sure in this game if you like Kirby.
And trust me, Jigglypuff is Jigglybuff in this game- not weak at all...