Here are my initial thoughts (as a not Dedede main):
You can't let yourself get suckered into hitting away on his shield. D-Tilt and Shield Grab are good options out of shield against Charizard, however, his jab also has more range than ours and hits as quickly. Keep in mind, Dedede's jab range is very good! The first is longer ranged than our Jab 1 and Jab 2, and the second has about the range of our D-Tilt. Remember that the second hit has more range, and that both hits draw you in! The fact that he can also grab you and use D-Tilt in that case makes close range combat with Dedede very difficult. Also launches you up at an angle where he can follow-up with F-Air and U-Air. Don't try to challenge this because of the second hit. If you get launched off, I'd recommend grabbing the ledge instead of trying to challenge Dedede in the air. Note that Jab 1 and Jab 2 can get chained into almost any of his other normals on the ground, including a grab or a D-Smash if they feel threatened, so that sometimes not even your shield is safe. Be ready to spotdodge if necessary; If he seems needlessly close to you, in the range where your jab could punish him, it's likely he could be looking for this.
D-Tilt outranges us, if I recall. F-Tilt gives him a better pivoting option than we have in our F-Tilt, so be careful if you're chasing after Dedede. (Both our F-Tilts seem to have relatively the same range, his going a little farther and being quicker, but at least ours can be angled.) Dedede's only hitbox in F-Tilt is the hammer. I can't stress this enough, don't try to treat Dedede like you can outrange him like Charizard usually prefers to do! He's better able to keep us at mid-range than we can to him. On the ground, you should try to keep it safe and hold your shield up, and again, always be ready to spotdodge strong attacks like Smashes so you can punish him more efficiently. U-Tilt launches you up into the air and reaches in front of him, so be careful around that.
Now, while this may look like Dedede has the advantage in terms of normals, we need to also consider how fast the two are. Charizard has the advantage of speed in both the air and on the ground, by a large margin. Granted, you can't just dash into his hammer, but we could gain control of the stage a little better than him if we shield the gordo and respect it. Reflecting it is pointless as it's extremely obvious, and they'll sometimes even double reflect it. In terms of general attack speed, he also happens to win. D-Tilt is six frames I believe, F-Tilt is able to be used on a dime, Jab is a wonderful attack in Dedede's arsenal that keeps us away from grab range. A blessing that we happen to have is a normal dash attack, but I imagine we wouldn't get to use it much except on failed smash attacks or the Super Dedede Jump.
Dedede's pretty slow in the air, but his attacks are great and have a lot of range. B-Air and F-Air can cover diagonally above and below Dedede in each direction, with U-Air covering above him very well and acting as a kill move. D-Air can both serve as a spike and an on-stage killer. N-Air can be fastfalled for a quick landing, and easily stringed into D-Tilt as a fast landing option. N-Air can even quickly lead into a D-Smash if you don't react quickly. Look out for him fast falling his aerials in general; Their huge range makes it easier to do this with than a lot of other characters. He can even condition you to shield against falling attacks, You have to look out for B-Air. It can kill Charizard at 120% near the edge if you're not careful, and at 110% (or lower!) if you're on or past the ledge. The same goes for D-Smash, and Charizard dies from U-Smash at 120% on FD. Basically, you need to be extremely careful. Just be happy that our U-Smash is overall faster.
I doubt Dedede would want to challenge Flare Blitz or Fly off-stage, but with us not using those two moves, I believe he'd dominate us with his two extra jumps and longer range, where speed doesn't matter as much, at least, if we're getting edgeguarded. I don't, however, know how much mileage we could get out of edgeguarding him besides Flamethrower chip damage.
Then, there are the Gordos, which most definitely aren't useless... But I don't think they'd be very helpful against Charizard off-stage, unless you recover literally right next to the stage where Gordo easily can hit you. Fly allows him to recover from beneath the stage on some stages, and with Flare Blitz or Dragon Rush, to a lesser extent, he can recover horizontally. However, when on the ledge, an Up-angled one can trap you into a few options and let the Dedede better read what you're doing. Jump up? You could get hit by the Gordo. Stand-Up? Gordo. Wait there? The Dedede forces you to act... Again, shielding them is the best option. Sure, it can leave you open to a grab, but you can spotdodge those.
Overall... This match-up does seem to be in Dedede's favor, but not hopelessly so. While he outranges us, something that's hard to adjust to as Charizard, he has relatively few options when we happen to be below him, especially since anti-air is one of our strengths in the first place. U-Air, in particular, as it only hits above and behind itself? I could see it being a decent damage dealer and even a kill move at high percents. Add Flamethrower for spacing, N-Air to force an action from him, (F-Air can be airdodged) and our speed still allowing us to dodge fairly well, I think we can potentially manage a win from Dedede if we honestly do play with everything we have.
