You play too aggressively. ROB's ftilt and fair will beat everything jiggs has except for pound, so if he just sat back and ftilted when you were about to come at him he would own you over and over. You use pound way too often and too predictably. Against a better player you would get wrecked because of it. Never pound while standing on the ground. Never dair into the ground because it has a ****ing hour of lag. You should grab a lot more. Heres how you should be approaching (copied and pasted from the Jigglypuff Discussion Thread)
Approach
Your best approach in most cases is going to be poking them with bairs. Before 30ish% you just want to poke them by spacing your bairs well and retreating in the air after it hits, until after 30ish%. If you follow up a hit before 30%, they will be able to hit you back before you can get another bair or shield. With doing single bairs, you should learn the timing to auto cancel the bair.
After 30% you can bair and then either choose to back away or you can follow it up by staying in their face. If you stay in they're face then you have a couple of options, you can either do another bair or uair in the same sh, or you can land and shield. If you land with another bair or uair, you can usually follow it up with more aerials, just watch out for their attack or airdodge when you try to follow up your hit.
Basically you need to switch it up and read what your opponent is going to do. If you take the hit them again route they can shield the next hit and then do what they want, if you take the shield route then they can hit you before you get your shield out. As long as you keep them guessing and can read what they are going to do, its smart to advance a decent amount. If you hit they're shield, do not advance.
Another good approach is to run and shield, and then react to what your opponent does. Jiggs has one of the shortest initial dashes in the game. Its good to use against characters that out range you and you can keep in their face and keep the pressure up with it. watch out for grabs. If they expect you to shield, they can just grab you, so keep switching it up using grab and the occasional dash attack.