Dedede's weird. Fundamentally there isn't really anything that Pit has to worry about, but if Pit does slip up, he can get hammered into next week. I know quite a few people (myself included way back when) swear blind that Dedede is a nightmare. It's by no means unwinnable, but it does require patience and a proper understanding of the matchup.
I think Dedede's biggest advantage is the fact that his disjoints beat out Pit's. Pit can space well with his aerial disjoints, but that's kinda difficult when Dedede's aerials have better range, hit harder and have relatively minor endlag. In addition, the fact that Dedede gets great reward off of a grab means Pit will seldom approach unless there's a very big opening to exploit. We can't afford to rush into this match. We have to be careful and force Dedede to play the angel's game.
Pit's combos are useful up to a point. Dash attack comes out quickly and can punish Dedede if his guard's dropped, which can combo into u-smash at low percents or f-air and u-air at higher. He's got the usual grab -> d-throw tricks (combos into u-smash and potentially all of his aerials), but Pit wants to stay out of Dedede's immediate space and hence a grab is seldom a realistic option except as a very coy punish. Dedede's big hurtbox should in theory make him combo food, but his floaty stature and ability to punish Pit with n-air or f-air means Pit won't get much of an opportunity to rack up too much damage in one fell swoop: it needs to be concise and consecutive. We need to go for small increments of damage whenever possible instead of going ham; since Dedede can survive well into the mid-hundreds and just gets stronger with rage, we need to do as much damage as possible before we even think of going for the kill.
Pit's arrows can be useful if used correctly; don't write them off just because their damage output is weak. If Pit's out of range (which, if he's clever, he will be) and Dedede is daft enough to try pressure him with Gordo, fullhop arrow can reflect it back with a bit of charge with relatively small risk involved. This is something Dark Pit can't do, so it's worth bearing that in mind. I'd still be wary of using Gordo in neutral, since it can be very easily intercepted and used as a punish in itself.
Quick tip: Pit's n-air can't reflect Gordo except on the last hit, because only the final hitbox does sufficient damage to reflect it. This doesn't really work on a Pit that even halfway understands what they're doing, because they'll just use something else to knock it back, but it does make for a good party trick if you're up against a fledgling angel~
Killing Dedede is...oy vey. Pit doesn't really have trouble killing, I just want to nip that misconception in the bud: he has trouble killing consistently. F-throw, f-smash, u-smash, tipper f-tilt and tipper b-air can kill pretty neatly in theory, but with Dedede's weight and the aforementioned difficulty of getting any of these hits to land, execution doesn't match up to theory. We want to make sure Dedede is reasonably certain to die before we can go for the kill confirm, so that means we need to rack up as much damage as possible whilst minimising it ourselves. I usually wait for Dedede to drop his guard and then go for fresh f-throw at the edge or, if I can condition Dedede to go aggro, bait an unsafe move out and then punish it with a sweetspot b-air. If we can survive to the extent that we've got more damage on you than you have on us, we can bait the penguin out and wait to exploit the chink in Dedede's armour. It isn't ideal, but it's serviceable.
The issue here is that Pit can die at any time if Dedede connects a hit, so even a tiny mistake can be fatal if Dedede gets momentum going. With momentum on his side, Dedede can become an impregnable stonewall and toss Pit about like a tiny winged ping-pong ball - especially since Pit has difficult landing, so the threat of u-air juggles can force us to go for the ledge instead of trying to come back down on-stage. Guardian Orbitars don't present nearly as much of a threat as they seem to in theory, since the frame data is disgusting: by the time it's deactivated, Dedede can just do what he wants to us. I don't recommend it, personally, it isn't something you need to worry about.
Dedede's very difficult to edgeguard. His multiple jumps and invincible recovery means he's probably going to get back to the stage if he can get momentum going, plus his aerials can challenge anything Pit serves at him. If Pit's fast to react and Dedede's at high percents, he can pressure Dedede off-stage with f-air strings, but d-air / b-air spikes are very difficult to connect safely. I think he's got an easier time edgeguarding us, since Gordos can be a bit of a hassle and walkoff n-air / f-air can force us to weave about, but Pit's recovery means he can just go deep and recover under the stage without much to worry about.
That said - and this is the main reason why I don't write this off as a clear win in Dedede's favour - there is a fundamental flaw in Dedede's gameplan: his disadvantage stage is very poor. Pit may get battered about if he makes a mistake and he can't get too intense for fear of his combos being beaten back, but if we can get momentum going and condition Dedede to use unsafe tactics, we can keep the pressure on just by virtue of our talent for playing on reaction and varied toolkit. We can bait the approach with retreating f-air and shorthop d-air, or even fullhop arrows from a distance, since Dedede's only projectile can be used to punish him and he will ultimately have to come to us if we just pester him from a distance. If we can find an opening and exploit it, we can get decent mileage out of n-air (a missed tech from it can setup for a dash attack or jablock) and d-air (combos into itself and can kill at high percents). While we can't get too brash with f-air strings and u-air juggles, we can definitely put the hurt on if we force Dedede to come for us. Pit may be weak and small, but he's wiry and we know how to play hit 'n' run if it means we can seal the stock eventually. We will play for the big picture, because that's the only way we can safely win.
It's a difficult matchup in theory, but it's very possible for Pit to win just because his playstyle means he benefits from good fundamentals. We just can't afford to make too many stupid mistakes or we will get punished hard for it. I'm gonna say it's even, maybe slightly in Dedede's favour because of his ability to end the stock with a good read, but I don't think it's a nightmare for either party.
EDIT: I don't really consider matchup ratios very often, but for the sake of community data, I'm gonna say it's 55:45 to Dedede.
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