when you're at high % absolutely never land on top of mario, try and use mobility to get on a platform instead of landing on the ground because he can usually punish those landings every time. careful with the top platforms at some %s because mario is capable of zss'ing diddy sometimes. if you're on the ground and ended up stuck in shield at a high % but he can't kill you if he grabs you, just hold the shield. don't roll or spotdodge because he can usmash read those things, but all he can do is grab if you shield. the more you force mario to be unable to kill with usmash the more you frustrate them and you can double or triple your stocks lifespan if he has to kill with an aerial or a 180% backthrow. he'll take you to danger %s fast with his combos, but if he can't kill you then, it doesn't matter.
yoshi idk eggs explode if they hit anything (they are basically flying mines) and you go under them if you dash attack sometimes but idk about every time. they really like to b reverse neutral b because it's a really good command grab so it's not going to be easy to landing trap yoshi with typical banana stuff but you can try to intercept him instead sometimes. he has bad startup right beneath him, so uair is good at juggling but you need to watch for down b. his recovery sucks so if you ever see a yoshi player pushing their luck with eggs onstage and not grabbing ledge you can weasel your way through that sometimes and bully him back off. tbh i find gimping yoshi easier with fox than diddy since fox's bair has the knockback for it but diddy can do it too. be prepared to airdodge fairs when you're offstage at every moment, since a lot of yoshis go for it. footstooling him if he's spent double jump kills him every time lol