I'm going to post from personal experience, not really any real evidence put into this.
To me, Kirby's worst matchups are (if you're playing an equally, or better skilled player):
Metaknight
Pit
Ike
Marth
Sonic
Fox
Falco
and if the player is good enough, Ganondorf
The first two can fly just as good as you, and it makes it really difficult to edgeguard against them. Kirby's style of of play requires you to get up close and personal with your opponent, which is pretty much a suicide note against a skilled Ike. Sonic is deadly fast, and is a huge pissoff, if you're playing a good one. Fox and Falco are pretty much the same as Sonic, if not worse. And Ganondorf is just a powerhouse, but most of the time you can outplay them if you're good enough.