Honestly, the Diddy matchup isn't quite as painful as the Shiek MU anymore, especially with the buffs Zard has gotten as opposed to Diddy's nerfs. At launch, I would have certainly agreed that the Zard/Diddy MU was at best a 10-90 for Diddy; all of Diddy's options made Zard a cakewalk for him. Now that Diddy's been so hardly hit by the nerfhammer while Zard's been getting buffed what seems like every balance patch, Charizard definitely has tools to beat Diddy now, but it's still not quite even. Maybe something like 40-50 in Diddy's favor is a good statistic.
Diddy definitely has enough options to be a threat to us, but the nerfs he's gotten have reduced his effectiveness in the MU, in my opinion. Flare Blitz deals handily with Diddy's bananas, and Flamethrower is also good at negating Diddy's stage control by shutting down his ability to toss bananas at Zard for an easy grab combo. Diddy has to respect Flare Blitz, as well; because it's so powerful at shutting down his most viable approach option and can be really painful for him if he can't react in time. The loss of Hoo-Haa's effectiveness for Diddy makes it so juggling is less viable for him as an attack option; especially since now Zard's aerials come out faster and are stronger than Diddy's nerfed UAir, which was once one of his most effective kill moves against Zard. Even still, though, Diddy has the advantage in that most of his moves are able to combo into each other, which can make it harder for Zard to predict him and adapt against it.
Zard's jab is really quick, and his grab is also pretty fast, as well; add to that his combo throw and aerials that are quick to come out and I honestly think that Charizard is pretty decent at fighting Diddy now. Charizard's NAir can also cover a decent amount of Diddy's approach options in the air since, again, Diddy's UAir got so horribly nerfed; which means it no longer works as well as a defensive option for Diddy against Charizard's aerials. Charizard's kill throw also kills Diddy pretty early, IIRC; not as early as Puff, but Diddy doesn't live as long against it as DK, either. Zard's fast grab speed means that even if he whiffs it, there's not much time for Diddy to punish the whiffed grab.
Flamethrower also shuts down Diddy's Rocketbarrels pretty hard as an edgeguarding tool, especially if Diddy is trying to recover from below the stage, which is a pretty bad thing when that's his only vertical recovery move. Monkey Flip also gets shut down by Flamethrower, though that's a lot less easy to do since usually Diddy will use Monkey Flip if he's trying to recover above the stage; so there, I'd suppose either Nair, Uair or Fair could do the job, maybe Dair if you feel like trying to spike him. However, Zard still can outlive Diddy due to his higher weight meaning he's KO'd at higher percents than Diddy; and a Charizard with rage is a scary prospect for Diddy, because it makes all his viable KO options even more ferocious against him.
The fact that Zard's DAir comes out so fast also makes recovering with Diddy a pretty big risk; do you go for Rocketbarrels to get vertical distance, but risk being open to Charizard either coming in and stomping you to the blastzone or simply detonating the barrels with his Flamethrower and watching you fall to your doom, or do you try to go for Monkey Flip, which will make Charizard go in with his own aerial to counter it and could still end well for you, but doesn't do very well in vertically recovering? Hell, even trying to use Monkey Flip above the stage can be risky if the Charizard predicts it and uses Flare Blitz, which outprioritizes Monkey Flip. Diddy does better in the neutral, but Charizard is definitely better at the offstage game, and is great at keeping Diddy offstage. The fact that Flamethrower and Flare Blitz both cancel out the banana, which is Diddy's bread-and-butter in neutral, is a major factor for the MU. Without the banana in place, Charizard can have a much easier time with Diddy, since bananas are almost a staple of Diddy's neutral game.
It's really quite close now, even if Diddy still has a slight edge over Zard.