Aaaaaand we're back to discussing Diddy.
There are going to be a million characters in this game with claims to beating Diddy once people train in the matchup, but most of them will be ignoring what the Diddy side can do and just focusing on what their side can do. The Diddy player has a brain (arguably) and a controller in his hands, so he's going to see what's happening and respond to it. Diddy isn't a one-dimensional character; he gets much reward from grabs but he does have an entire kit besides dash grab and up-air, and he will use it to condition you into putting yourself into dash-grabbable positions.
Whenever possible, we should be thinking one step ahead and predicting how the opponent will react to the strategies that we're pulling out. Definitely not saying that the Megaman boards aren't doing this (I don't know; I haven't been there), just pointing it out as a possible trap.
On the subject of Megaman, sure, pellets neutralize peanuts, so Diddy won't be relying on camping Megaman out. Instead, he will be trying to get in past the projectile barrage and land a grab. Megaman's close range options are not that great so Megaman has to try to use Leaf Shield and safe midrange zoning tactics to keep Diddy out (Megaman's aerial game is really good; f-air and b-air especially are dominant options). Is this effective against Diddy? Maybe. I don't really think so because Diddy is really fast, and he can mix up side-bs and banana tosses at appropriate times to still land grabs/get Megaman in the air (or just in close range), where he's less good/safe. His f-air is still a force to be reckoned with, and d-tilt pressure can be quite intense when he does manage to get in.
In the air, Megaman doesn't really have a reliable way to land against up-air, at least I don't think he does from my limited experience with the character. D-air is really easy to avoid and he has no fast n-air interrupt. Does Leaf Shield in the air protect against attacks?
When Diddy is offstage, Leaf Shield only gimps if the Diddy tries to recover from directly below. Megaman does have a really solid way to punish side-b to the stage, though, with his f-smash being as awesome as it is. There's not much a Diddy can do against a charged f-smash into a Leaf Shield. So I'm guessing that Diddy will try to recover high first, in which case Megaman does have that up-air to catch him out, but that won't send Diddy offstage again so unless it kills it might not do that much good in gimping. Seems like it's safer for Diddy to just recover high since recovering low can be risky against Megaman. So that's still pretty good for Megaman, at least: a Diddy who always recovers high (or most of the time, with some low mixups) will be taking damage on landing, and at higher percents getting knocked off again while trying to get back to the stage.
On the flipside, Diddy can't edgeguard Megaman as well as you would think, so overall, just based on my initial thoughts, I'd say that the matchup is pretty doable and at this stage I'd call it even. I don't really see how Megaman wins, though. D-throw to up-air / f-air didn't stop being ridiculously good or anything.