I disagree, D3 imo is an old trick that while is still solid,
does best as a wall to shut out bad characters. Coney, the last D3 to reach a top level game, was still limited by the fact that he could not beat top level MKs. Lucario has at least taken sets off of/won against good MKs like Atomsk, Tyrant, Lee Martin, etc.
Plus the results don't lie, especially over an extended period of time. I have never heard of a D3 beating a Falco, so while it's theoretically doable to what people say, I have to look at who grossly outcamps and who outdamages, that's a MU that to my perception, is borderline countering to D3. Pikachu is to a lesser extent, although I'm sure ESAM bodies CO18 whenever they play.
I totally disagree with DDD v. Diddy, people really, really underrate Diddy's camp game nowadays. It doesn't work against characters that have a semblance of a projectile (like Lasers or whatnot), but against characters with poor or no projectile camp game to speak of, it's really hard to get in on Diddy, and Diddy's punish game isn't even really read-oriented, it's almost reactionary how he can play (pretty broken imo), his shield options are so good just because of what a banana throw can do. It's a pretty even MU in my opinion, Coney said it was in Diddy's favor before he quit. Also, D3 could have an air game like peach, if he:
-had an aerial that was completely safe on block and could effectively pressure at any given time (peach fair)
-had a good gtfo aerial (dair/nair)
-had better air mobility (he's got multiple jumps, but he doesn't have the space control Peach has with floating)
Overall, D3's aerial game is more of a traditional zoning kind, but it's also more of an edgeguarding type (bair is really good at bodying recoveries for lots of damage, I can give it that).
I have also seen a positive record in Diddy's favor against D3 from Gnes, Lie, Felix, and ADHD. Bananas stop CGs pretty well as does Diddy's overall camp trap (banana in front, holding one while using his popgun). I probably have a little Diddy bias because we have a really nasty Diddy player in our region (Felix), but that's how I see it.
idk, I have a hard time believing D3 is all that great when I live in a region full of them and get some close games with characters that he's supposed to "body". I even got a really close match with Seibrik's, which is supposed to be pretty good. He's imo the kind of character to round out and stay near top of B, maaaaaaaybe bottom of A when MK is gone, but he's basically there to make sure no crazy low tiers that happen to have decent MUs with MK make it out

.
Snake and Marth are pretty even I will say, Marth might actually be in D3's favor, it seems really hard for Marth from what I've seen. That being said, there's still a lot of good characters that beat him hard/borderline counter him, because in the MUs that he does have to approach, it also sucks that his only big things are like dash/walk -> shield mixups, spotdodge, and bair walls, with the occasional swallow/dair. All of which are pretty doable for characters that camp him hard like Olimar, ICs, Falco, etc. Not to mention MK is like one of his worst by far, and borderline unwinnable.