I hope you don't mind reading a lot. =/
It would help to see some videos of you playing, but I'd understand if you can't (I don't own a video camera).
You are dealing with King Dedede. He is heavy yet possesses a very strong air-game. Don't neglect using your air-game. A decent combo to set up a U-Tilt is a Fast-Falled N-Air leading to a U-Tilt. It works and is very useful.
Once you have them in the air, try juggling them. However, expect an air-dodge. Once, they're in the air, get under them and wait for them to air-dodge. When their air-dodge is about to end, hit them with a U-Air. You can try to continue this for a while but they'll learn that you'll wait until they finish air-dodging and instead try drifting away or attacking. I suggest at one point switching to a B-Air to do some real damage.
It helps to gauge your distance when throwing Waddle Dee's. If they are far away, fully jump and throw a Waddle Dee. If they are in an awkward not-close-but-not-far area, short-hop and throw a Waddle Dee. If they are Sonic and trying to run at you, don't jump at all and throw a Waddle Dee.
If they are running at you expect one range closer than they are. If they are far away but running, short-hop and throw a Waddle Dee.
However, I don't advise spamming Waddle Dee's. While great projectiles, they are slow and predictable if used too much. Use Waddle Dee's to return from getting sent flying and to disrupt your opponent. Also use them as projectile buffers. Samus is screwed if you've got a Waddle Dee between the two of you. And none of the Landmaster Space Animals' Blasters can reach you if you've got a Waddle Dee between you two. USE SPARINGLY.
As for your F-Tilt, use to keep yourself distanced. Don't use it to fend a charging attacker off unless you know that they can out-range your D-Tilt. Your F-Tilt also cancels many projectiles. REMEMBER THIS.
Your D-Tilt should be used very often unless the foe likes to jump into the air and attack, at which point you should use your U-Tilt. Your D-Tilt has decent knock-back, decent damage, and is a fast move with little ending lag (I, personally, like to fake my foe out by using an F-Smash [Which I know they'll wait off and charge afterwards] and then as they charge me, quickly launch off a D-Tilt. It usually catches them off-guard). It also has great priority so use it often as a means of attacking charging foes.
Use your U-Tilt multiple times to juggle when your opponent is at low percentages. It helps build damage quickly. ^_^
I don't chain-grab much because I'd rather follow a D-Throw with an F-Tilt or just B-Throw to build damage quickly. Whatever works, though.
D-Air is a great move that scores multiple hits and can lead into some decent combos. Use it for a counter-approach or an approach in itself.
Learn to WOP, learn when to Dededecide and when it is practical and how to trick your opponent into falling for it, learn to play mind-games and play some ledge-games as well, and find someone who will take you in as a Dedede apprentice. Someone who will teach you things only a Dedede player can teach and something that will help in fighting other Smashers.
I find studying someone else's Dedede is a very good way to get better yourself. The more you watch how a pro Dedede player plays, the more you'll tend to do some of the things they do and create your own strategies off of them. If you don't find someone who will take you in, YouTube can help learning how pros do it.
Just keep practicing and you'll do fine. Develop your own strategies, learn mind-games, learn to pressure your foe into doing something they don't want to do, and just have fun with him.
He's King Dedede. You can't go wrong if you practice.
I hope it helps. ^_^
