I say 55-45 Ness, Dedede is a huge target and is easy to PK Fire and Tailwhip. Recovering isn't that bad, considering Dedede is far away. If Dedede is close up, unless you're being risky, use the magnet stall recover, and Fair or Nair him if he gets too close. Bair is a pain, but I think Fair has more range. (Keyword: think. ^^; ) The chaingrabs are BAD, try to get a stage he has a hard time chaingrabbing on, like Norfair or Corneria away from the fin. Dtilt has annoying range, and leads into other things, but perfect shielding helps a bit. Try to distance yourself from Dedede, his projectile is annoying but Waddle Doos= Free Healing, and Waddle Dees don't do anything but make a wall of frames infront of Dedede, if he runs into them that is. ^^;
EDIT: Ref, isn't he easy to spike and his recovery easy to Tailwhip? o.o