ONE
0:07 - wtf? lol
Learn how to instant catch + instant toss items on the ground. Especially for this matchup, but it's just incredibly useful for your piece game.
0:51 - Why do you getup attack? Jumping back and FFing onto stage is a much faster option and covers more options against a recovering Diddy.
Fast fall to the stage more often.
1:29 - Work on your hitconfirming, you could have gotten a second dsmash easily. Also the optimal dsmash combo is dsmash x 2 > fair, for 39~%.
1:32 - Don't side B when they're approaching and you're stationary. It's asking to get hit out of it.
1:35 - Try to go for the edge when recovering. Seriously.
1:45 - bair???
2:53 - dsmash out of dash attack does not work at < 60%.
3:46 - 4x trip.
You backroll way too much. Try short hopping away, or SH uairing away.
You tend to double jump when it's really unnecessary and you're losing opportunities to punish because of it.
TWO
1:09 - You could have edgeguarded Diddy easily, if you had simply left your tether dangling, and retracted when you heard the Diddy release his upB. You get extra invincibility from it and you would have either gimped him, or put him in a position to punish.
The roll edgeguard does not work. This is not Melee.
Try landing with nair instead of fair. Autocancelling is always desirable and it's a very good shield pressure and combo starter move.
1:20 - Work on your dsmash.
Try to fast fall your aerial approaches.
Practice your nair spacing in training mode. Make it pixel perfect so that you don't miss your opponent, but you're not so close as to get shieldgrabbed. Nair is amazing when used right.
2:33 - jump on stage sideB is generally a very bad option.
THREE
0:59 - dsmash out of shield? try dtilt.
3:37 - fair onto stage is also generally a bad idea. You want to either uair them to get rid of their pressure, downB off their head to get away, downB kick them if they don't want to shield, jump FF airdodge, paralyser downB kick, sideB off stage, or jump off mixup.
I think your ZSS needs a little work. Your bair spacing is generally nice, but you do some weird, not-optimal things. You also make some bad decisions with sideB and usmash use. You also very rarely use dtilt, which is weird because it's one of our best moves. Try empty shorthop dtilt. Safe, profitable and mixup-able.