Me and V are 1-1 btw, once in tournament and once in MM. I'd like to play against more ZSS's that aren't DRN who knows how I play before I say the MU is definite, but I'd say the MU is very slightly in Diddy's favor, but I've been starting to lean towards even lately. I'd say ZSS is definitely top 3 in the game with bananas, she has the same frame data as diddy's item throws with a long glidetoss and moves such as dsmash that are almost perfectly safe on shield.
I feel like the Diddy-MK MU is almost -2 (keep in mind Apex ruleset), but only if MK plays the MU right/perfectly which no MK does cuz they're all bad.
IMO, Diddy goes even with ICs, loses to Olimar, goes even with marth, loses slightly to snake but it's almost even, loses to falco, pika and ZSS are probably about the same difficulty maybe ZSS slightly harder. Pika's platform control/pressure on Diddy is really good so it's kinda stage dependent. Diddy loses to wario, and this might sound kind of absurd to some people, but I think he loses to Pit (still keeping in mind with Apex stage list) if the Pit goes for the timeout. Pit's run away game from Diddy is really good and when he does come into those situations where he has to fight diddy his multiple jumps + palutena wings helps him stay off the ground away from bananas and his frame data is pretty fast and can definitely compete with diddy up close.
I think Snake's MU spread is pretty close to Diddy's, I think they could probably be interchangeable. I think Snake's MUs are a bit more polarizing than Diddy's, Olimar definitely beats Snake harder than Diddy and then Pikachu can be argued for -2.