This match-up is a pain, very even but a pain. First things first, Dedede does have less than good frame data, especially against a combo heavy character with fast attacks and mobility, but Shulk isn't one of those characters. After comparing the two
here, it's apparent that Dedede is actually able to match a lot of Shulk's attack speed. In fact the only moves he has that are really slow in comparison to Shulk are Jab, Dash Attack, and F-Smash. Every other attack is pretty even in speed, with Dedede often being a frame or two faster, but unlike other characters that out-speed Shulk's attacks (almost every character in the game), Dedede only has slightly less range than Shulk not counting Dedede's N-air and will hit you for a lot often for approximately 10% per move, again not counting N-air. This means when you are trying to hit him, you have very small room for error cause one over-committing aerial and Shulk's eating a move that does +10%, which will hurt even more if Shulk has Jump, Buster or Smash on.
Thankfully, Dedede's movement speed barring his fall speed is pretty slow, so Shulk can just dance around him with Speed and Jump (just don't get hit in Jump it will hurt) From the Dedede's I've played, they like to hover right outside Shulk's jump range and take advantage of their fall speed to punish any attack Shulk tries to throw out with a falling N-air or B-air, or possibly force an action with a Gordo. If the Shulk just holding shield waiting for an attack, Dedede can just eat you. In this situation you either want to keep track of his jumps and try to shut down his air game with a U-tilt and force him to stay grounded. You might also be able to throw off his spacing by turning on Jump and try to space with an aerial, but messing that up can hurt. A lot.
Next annoying part about this match-up is Dedede lives
forever. He's very fat and unlike the other fatties, he has an amazing recovery with 4 jumps and an Up-b that is both invincible and snaps to the ledge as he ascends. Which means Shulk loses out on his great edge-guarding in this matchup, and many attempts to do so usually just result in free damage or maybe even a KO for Dedede. However this doesn't mean don't hit him when he's offstage, just don't go hard. Wait for him to get near the stage so he can't catch you from far away, and don't fight Dedede when he's recovering low cause Dedede can just U-air Shulk if he tries, which won't KO at stage level, but it won't get Shulk anywhere closer to winning. The only time Shulk can really for sure get a solid edgegaurd, is if Dedede drops from the ledge and opt to regrab, win which case you can either go for a dair or possibly an angled F-smash. Just watch out for if they bait it and hit you with a U-air.
So to KO Dedede at a reasonable % you will either have to land a clean Smash Attack (which needs a hard read or punish), turn on Smash (Risk factor), or land a Vision (Need a read or good reaction time). Thankfully, provided Shulk plays safe and doesn't get hit offstage, Dedede will also have trouble landing a KO. Dedede doesn't have a kill throw, and his KO moves are either slow or not as big as his other moves.
Another thing I think is annoying about this match up is that the Monado weakness become very apparent here.
Since Dedede hits so hard, Jump, Buster and Smash weakness becomes much worse for Shulk, since Shulk will eat huge amounts of damage if he's hit, and messing up in Smash is far more dangerous vs Dedede than other characters. It also means that the mobility Shulk loses in Shield might not be worth it. Because Dedede survives so well, Smash wont kill very early and Speeds damage nerf will become much more of a factor here.
The quick and short version
- Shulk out-speeds Dedede in terms of movement
- Dedede hits hard.
- Shulk will have to land way more hits to get Dedede to KO % than Dedede will.
- Attacks the players should be using in neutral are about the same speed and range, with Dedede's usually being slightly faster but with slightly less range.
- Because of the above point, both players will probably play super patiently and wait for a proper punish or make a read.
- Properly placed and timed U-tilt will shut down Dedede's on-stage air game.
- Dedede edgeguards Shulk way better than Shulk edgeguards Dedede.
- If both players play safe, KOing requires a good amount of work from both sides or a big mistake from one player.
- Monado Art flaws become more apparent.
Dedede can hurt Shulk really bad if the Shulk isn't as patient as the Dedede, but Shulk has better mobility options and with Monado Arts he can out-mobilize Dedede until Dedede messes up and go in for some damage. Both characters are really good at responding to each others mistakes and options. I think the match up is annoying and mentally straining, but it's even.