Well this thread sure sprang to life while I wasn't looking.
Re:
This matchup always surprised me with how frustrating it can be. Mario's end lag on everything is so low and Dedede's startup on everything is so high, and fireballs can keep Dedede out so well, that Mario feels like he controls the match most of the time. On the other hand, Dedede outranges him massively and can edgeguard his poor recovery easily.
Dash into shield to approach through fireballs, use (pivot) ftilt/sh bair/fh fair to stuff his approaches. All of his aerials
should be shield grabbable, but be wary of the low landing lag on nair and the nonexistent endlag on dair (unsure of how quickly he can nair after dair; might be a frametrap?). I'm always highly wary of Gordo use in the matchup since Mario's likely to reflect them without even trying with fireballs and nair.
It's worth noting (unless it changed in 1.0.4, pretty sure it didn't) that Mario can't Cape Gordos. Of course he can reflect them with other hits, but it's a piece of matchup knowledge they may not know about, and it saves you the trouble of worrying about reflected Gordos with amplified damage.
Feels the matchup is slightly in Mario's favor. I'd like to think Dedede's range and potential for low percent edgeguards may mean it's even but idk
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I find it really funny that I used to think this matchup was slightly in Dedede's favor because of how easily he could kill Luma; playing it from the Rosalina side of things, rapid jab and uair/utilt/usmash just ruin Dedede's life.
Spaced autocancelled bair is fairly (completely?) safe on shield and tends to dispatch Luma effectively without putting yourself at risk of being shieldgrabbed; dtilt, dsmash, jab combo, and dash attack will all get the job done too if you have an opening to use them. When Rosalina loses Luma, she loses a lot of her disjoint as well as nearly all of her kill power, and her damage output and combo potential likewise suffer for it.
Offstage, Rosalina's upB is highly vulnerable to edgeguarding, though her recovery speed and distance lets her mix up whether she goes for the ledge or across the stage/to a platform. In Dedede's case, I don't believe any of her or Luma's aerials hit hard enough to break through his upB super armor, her spike included, so recovering low is the way to go; recovering high gives Rosalina the opportunity to punish with uair juggles, and at 10+4% each you don't want any part of that.
Matchup jank: Because of the massive hitlag when Gordos hit, any Gordos that hit Luma will hang in front of Rosalina's face for awhile and then rebound. The Gordo will hit Rosalina for 10% if she tries to run forward through it in this state.
I feel Dedede wins without Luma around, and Rosalina easily wins with him. Maybe 40:60 if I had to slap a number on it, but I'm not feeling too confident in the number.
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As far as projectile characters go, Samus is the only one I've never felt shut out by. Her end lag on projectiles is fairly substantial and she can't drift away while firing them like Yoshi or Mario can, so (power)shielding through them to approach isn't too rough. Once he's in, he can slap her around with his disjoints.
I don't have much to say about the matchup overall, but I don't feel like it's bad for Dedede.