The answer isn't just "Aircamp and you win". You have to be REALLY good at reading and understanding very subtle things for it to work against opponents who also understand how it works for Wario.
You have to understand your opponents patterns, you have to quickly compile a function/possibly just a regular formula involving the characters air horizontal speed, their rate of direction transition (how fast/how long it takes them to go from moving full speed left to turning around and moving full speed right in the air), Dedede's ability to Double Jump multiple times, fast fall multiple times, Double Jump in 3 general directions (Straight up, left, right), his hitboxes (attacking and defending), and this isn't including the data you need to compile for Wario involving some of the same facets of information.
There's fairly complicated math behind it, doesn't matter if you can crunch the numbers if you can't actually act out your ideas, so there's a good chance a lot of people won't be able to properly apply it to actual gameplay.
If you want to talk about simpler things, Wario does well on Battlefield, Lylat, Smashville, and PS1. Just ban FD in the set or CP them with someone like Falco or Olimar if they pick FD with Dedede.
Also OBM touched on something that is fairly important: After Dedede does a move, if you are right outside his range, you almost always can safely move in and at least poke him with Fair and then back out. Even on his Bair, framewise it's a trap because Wario can punish RIGHT after the hitbox goes away.