It's possible you're just doing "what every Ness player does". When you gain GSP, you potentially run into people that can deal with your level of Ness play, and when you lose GSP, you're once again faced with people you can beat up on.
For starters, I preface this by saying that online results aren't super important and you shouldn't stress over them. Ever. The difference between online and offline isn't night and day...but it's bad. Kind of like morning and evening in the winter time...so it's pretty damn close to night and day.
Secondly, if you can work on developing and polishing a play style with your character (Ness in this case), you'll find good improvement for yourself. I used to feel really ****ty about my Palutena as a secondary pick, but then I decided to say "**** it" and work on developing my play on her to address WHY I wanted to pick up a pocket character in the first place...which was to deal with zoners and lame out super heavies. Working that angle into my play, and working on removing online habits, helped boost my confidence/performance on Palutena so I can pull her out during the little tourneys I go to when I think Yoshi isn't cutting it, which sometimes happens.
Bringing this back to your case, though: When you develop and polish a play style, it can help with your confidence in the neutral game, and confidence leads to a more commanding performance. Idk if having a flimsy/undercooked play style is your problem, but it could be, especially since randoms online are, apparently, figuring you out too easily mid match.