Do you have any videos of your matches? Your character choice really is not that important, and at your level, Jigglypuff, while still a bad character, still has some fundamental traits.
I feel like your problem might not be your character knowledge, but your bad habits. Maybe you don't know how to fully capitalize off a grab and your punish game is lacking, maybe you aren't proficient at spacing aerials on shield and you constantly get shield grabbed, maybe you roll too much, maybe you don't mix up your approach and let people read you, maybe you don't pay attention to other people's habits and can't capitalize off repeated ledge options / tech patterns / etc.
It's difficult to give you specific advice if you don't provide a specific example of what you're having trouble with, nor provide a video, but I think you might be having problems with habits; it has nothing to do with your character.