If you don't win, then you weren't the better player in that match. It's as simple as that. It doesn't matter if you're 50-0 in tournament, if you lose a round or a match, for the duration of that match your opponent was better than you.
Dedede should have no problem at all against the ICs. His grab range is better, his chainthrow is better, his projectiles are better, his aerials are better, his smashes are better, his recovery is better. Everything about D3 is better than the ICs. Your problem probably stems from the fact that you have very little experience against ICs, especially chain-throwing ICs. Get them off the stage, gimp Nana in mid-air (which should be a piece of cake as D3), and you win. That's really all it takes. If the IC shield camps you to try and get a grab, grab them first. D3's grab is so much faster than the ICs, and you should have thrown Popo away by the time Nana can even react thanks to the desync lag.