There are several reasons for why Diddy does not lose to Marth off of the top of my head:
Agreed, it is even.
A) Diddy starts with the bananas
K
B) Diddy has multiple ways to out-range Marth
Diddy has one way to outrange Marth, which is fair. Which we massively outdisjoint. Move is still hella gay
C) Aside from inherent disjoint, Marth doesn't have any particularly strong advantage over Diddy
We **** him on the ledge harder than every other character in the game... The only character who has ledge traps even in the same tier as Marth is MK and arguably sheik. But Marth definitely outdoes Sheik, and I believe has better ledge traps than MK. Although, nado is really good at covering a lot of options at once. Additionally, Marth has 2 glide toss lengths. The one everyone knows which is very long. The other one, is much shorter, and very similar to Diddy's GT length. Marth can use many of the same banana tricks as Diddy, and has better moves to use out of them. Additionally, just as Diddy has his sideB mix-ups w/ a banana in hand, Marth's ability to use DB while holding a banana is highly useful. Especially considering DB routinely beats (or at worst trades) w/ diddy's flip. Overall, I agree with this, except the addition of ledge ****. I will admit though, outside of tipper fsmash, Diddy does have a better punish game on stage. (Oh yeah, Diddy gets 0-deathed off a grab @ 0
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D) Marth has ways to be initially safe (e.g. Fair on shield), but his actions after that tend to be very committal and require a much smaller window of error than Diddy. This is especially important because Diddy has options at neutral that allow him to react to Marth, whereas Marth is relatively limited to reads at neutral (unless Diddy makes a mistake).
Ehh, mostly agree.
E) Marth might have a slight nod to Diddy on ledge guarding, but Diddy's ledge trapping is clearly superior to Marth's in the MU.
Uhhh, no. Definitely not the case. Diddy's ledge traps are alright, but, nothing close to what Marth can do on the ledge.
I personally think that Diddy's lower tier MUs are largely skewed toward the lower tiers favor because of the difference in MU knowledge (apart from bananas). Diddy's Side-B/Fair/etc. alone are very powerful tools that most of the cast has great difficulty with. Even MK vs. Ike can even if MK wants to consistently try to challenge Ike's Fsmash with Neutral B.
I agree w/ the skewing. Although, your last sentence is ********, rofl. Ganon can fairly easily stutterstep fsmash MK's nado and diddy's side B (will trade iirc) if they both want to just spam them stupidly. Every character has an option to beat everything in the game if they just do it in a manner as ******** as MK just nadoing @ Ike when Ike can fsmash.