The only thing hard about jiggs is her spammable aerials.
Her immunity to gimps, combos, tech chasing, and edgeguarding make her difficult by cutting down damage potential immensely. The fact that her duck beats grabs is also hilariously amazing because it cuts down the most common answer to crouch spam. Meanwhile, she still retains the ability to death combo you, or gimp you, or kill you at low percents, etc.
The punishment disparity can be difficult to overcome. Functioning largely without a grab can also be difficult.
Every other move in her arsenal is meh, and has a counter.
Aerials are how she racks up %, sends you off stage, gimps, everything.
Even if her aerials are her best feature, and form the basis of her bread & butter game, that doesn't mean the rest of her character is "meh". Many of her traits are fantastic. If Jigglypuff didn't have her duck, her Rest, her B-throw, and her F-smash, I assure you Sheik would hard counter her like you wouldn't believe. It's the combination of a whole lot of her gay stuff that makes her hard for Sheik. Not just aerial spam.
What else does puff have? Her smashes are mediocre, her dash attack doesn't set up for combo's, her shield pressure is whatever, her edgeguarding & gimping is good, but usually the reason your off stage in the first place is her aerials. Her grab game on everyone but the spacies is meh.
Puff's F-smash is beastly when it hits (duck --> F-smash vs whiffed move or CC --> F-smash is very underrated). It can also be used for edgeguarding to great effect.
Her dash attack doesn't need to setup for combos because it's a combo finisher and mainly used for sneaking under retreat aerials when not used to end a combo or tech chase.
Her shield pressure is awesome, you can't punish anything she does to your shield while she has the ability to attack it pretty much freely. On a character like Sheik, she can literally just duck in front of her and wait for Sheik to do anything and then go for a punish on it. That's amazingly good LOL. I wish I could sit in front of my opponent at various ranges and be safe from shield grabs, aerials OoS, U-smash OoS, and pretty much everything you can think of LOL.
Her throw game is situationally good. D-throw onstage can be very, very nasty if you can force Sheik to touch a platform (tech chase Rest or tech chasing with aerials can be gay). If Sheik DIs into the throw, she can just be combo'd outright. Near the edge both her F-throw and B-throw are excellent for setting up edgeguards or gimps.
Also, U-tilt. U-tilt is pretty good.