That's because Meta Knight has everything Dedede hates. He can lock Dedede down and all of Dedede's "Get the hell away from me" attacks aren't fast enough to shove Meta Knight off him. Due to his size, Meta Knight is also too short to get hit by up-tilts hitbox while standing on the ground in front of Dedede.
In the air, Dedede is still outclassed by Meta Knight due to f-air being slow as hell and b-air being out ranged. Dedede doesn't do well against fast multi hit attacks. Dedede also is the slowest moving character in the air (horizontally anyway) and that makes him easy to target by Meta Knight with up-b or aerials.
Meta Knight is Dedede's hardest match up. =/
According to people I've talked to at tournaments, Snake is Marth's hardest match up followed by Dedede.
My Creations in Modnation Racers! Check zhem!
http://community.modnation.com/en-us/players/IvoYaridovich/creations
