Sorry I was talking to my dad.
Basically what one of the problems is is that when you're in the air you don't dodge, which means people can easily hit you and get some combos in. And if you do get into a combo, if you don't dodge you'll get stuck until you get sent high enough to just drift out, but by then you'd have a lot of damage on you. Another thing is that if you use too many smash attacks you risk missing, which leaves you open for either a standard attack (pressing a basically) or another smash attack which can be more devastating, which is why I suggest using moves that don't take up too much time, or try learning some combos (e.i. the Ness one I mentioned: PK Fire traps the opponent, then a side smash launches them upwards a bit, and an up air allows you to knock them up (omg :x), allowing for more damage).
Then again it's a possibility I have no idea what I'm talking about but eh ¯\_(ツ)_/¯