From personal experience and observing tourney performance of other great DDD players, I find DDD's worst matchups in order to be:
1. MK - Extremely hard to edgeguard. Gimps DDD's recovery well with Dairs, Up Airs -> Up Bs, Up B's and has reliable horizontal KO moves his MK does not spam them (Nair, Dsmash, Fsmash)
2. Zelda - While Din's Fire isn't really hard to get past, it helps her control the pace of the matchup. Good Zelda's will not spam this move and use to make DDD approach a certain way and she can react to it effectively. On stages that are large and/or promote camping (e.g. - Castle Siege, Electroplankon and even Smashville), DDD takes risks going aerial and Zelda will likely rack up damage with Din's Fire, her aerials and a few smashes. Zelda has ton's of KO moves on DDD. Fsmash, Dsmash, Up Air, Bair/Fair, Up Tilt and Ftilt. DDD dies the best when he's hit with a move that has good horizontal knockback and Zelda has a lot of those.
3. Toon Link - IMO this matchup is still pretty even, but that only if you know how to deal with his projectile spam. Everything this guy does is a setup and he's testing to see how you react to his projectile spam like whether you are shield dashing, throw waddle dees and using tilts. His bomb is the projectile to worry about the most and will often help him set up Bair combos and just damage racking.
4. Pit - I think this matchup is pretty even, but it's ****ing annoying as hell. Again his arrows are there to position you a certain way. If the Pit is any good, he'll make this battle extremely long and mentally exhausting and wish you were playing Melee. His Fsmash is extremely spamable, but it you PS the first hit, you'll get a good frame advantage and you'll be able to grab him. However, I play very smart Pit's where they pressure the life time of your shield and then fsmash which is why PS is essential in dealing with it.
5. ROB - Slightly in favor of ROB if his defense is tight. Good ROBs will control the matchup with well spaced tilts, fairs, lasers and gyro blades (Down B) and he'll make you work hard for that grab. A lot of ROBs do not know how to gimp DDD's recovery and it's what makes this matchup slightly in favor for ROB.
I dont have too much problems with Olimar. That matchup is really tricky though and once you get more experience against the little guy, it's not bad at all. DDD's Dtilt and Ftilt are really effective in this matchup. You have to be good at PS and then releasing it into Tilts though. Often the Olimar player is spamming his over B and mixing it up with either grab or smash. Either way, you should buffer spot dodges into a move to play it safe and stay close and guess the attack correctly and PS -> Grab or tilt (depending on his position). If you get a grab off, Up throw and rack up the damage. Olimar has good aerials, but not a great aerial game. Yours is better and you want him there.
Edgeguarding Olimar is easy, but it tricky at first because good ones will stall with his down B which has some invincibility frames. Just be patient and how the timing when he's not invincibility and continue to knock him off and edgeguard.
You should love platform stages vs Olimar and you'll get several low percent KO's or Up Tilt scares from them. Of course, Olimar will continue to camp you and DDD can reduce that camping which the recommendations I prescribed above. Up Throw Olimar on stages like Battlefield and he should take a lot of damage. This matchup is at least 6-4 in DDD's favor if you consider all of the neutrals and counterpick levels.