Never EVER EVER EVER EVER take Diddy to SV or FD. SV is Diddy players' best stage. FD is Diddy's best stage, and neither of them are good for Pikachu in the MU unless you have RIDICULOUS item control (which, face it, no Pikachu has ESPECIALLY with his lackluster item throws). Assuming Apex stagelist, take Diddy to Frigate. If he bans it, Yoshi's. The key to the MU is fair, so Diddy camping under a platform isn't bad for you since you can just fair and it will go through pretty much everything (And it catches bananas VERY well). If you don't like Yoshis, BF is a decent choice considering the platforms make it difficult for Diddy to combo you if it starts under there. PS1 is normally a Diddy's CP assuming they can't go back to SV or go to FD, so don't take them there either. Lylat is meh becuase you can't get outside pressure with T-jolts well because of the platforms, but he can't really get you in landing traps as easily.
For Olimar...*shrugs*