http://www.youtube.com/watch?v=jx6fRmhzhgY
I'm pretty sure my mistakes aren't Pika specific, but I'd still appreciate any feedback :D
Still being helpful here yays.
Not speaking from experience on this one, but from general knowledge I've picked up playing as/against both characters:
Pika vs D3:
-Rule #1 of the Dedede Matchup: Don't get grabbed. He grabbed you a LOT. He didn't capitalize on it a lot, but he got a lot of free damage because you kept running or jumping at his shield. Ways to avoid getting grabbed include SH Dair onto his shield (as opposed to Fair), staying away from him while he's in shield, etc.
-Recover safely. This means as far away from Dedede's grab/F-tilt range as possible. I saw you get shieldgrabbed a couple times because you tried to get back up from the ledge with a F-air and didn't follow through with it. I'd personally go a little higher and use D-Air then go past him. A good on-stage example of you doing this was the OoS D-Air at 1:40. You hit him and he missed the shield grab.
-This is a tip I've gotten a few times from the R.O.B. boards, but it's still pretty applicable. Spam him out. If need be, bring him to time. Dedede is really difficult to kill, and Pikachu doesn't have the strongest (reliable) finishers in this MU. This is an issue R.O.B. runs into a lot too; our two kill moves (Nair and Bair) won't kill Dedede for a REALLY long time. Also, he can't grab you if you're safely spamming TJ's.
-Don't QA randomly. You SD'd twice because you QA'd off the stage. I'm not sure what you were trying to do, but whatever it was, it went wrong.
This one I can actually speak with some experience though:
Diddy vs D3:
-Learn to glide toss. It is literally impossible to play Diddy well without being able to Glide Toss and Pseudo-Glide Toss (or whatever the jump-cancel throw one is called). You definitely played my Diddy once or twice, and you probably noticed I slid everywhere, chucking bananas as I went. The timing isn't difficult at all (it's about as hard as a shieldgrab from a powershielded attack) and it becomes reactionary after a while. Almost every time you did a roll-dodge could have been a glide toss with a banana being chucked into D3's face.
-Learn how to control items. Probably the most important aspect of Diddy's game is controlling his own bananas. Things like instant throw/drop, Z-Drop, and Glide Tossing. 10 seconds after the match started, D3 had control of both bananas. This should not happen.
-Don't spot-dodge so much. Do an OoS Nair/Bair every once in a while to mix it up.
-Dedede isn't going to die at any kind of reasonable percent except from a Fair. So get him to a completely unreasonable percent by punishing everything he misses with GT > Grab > D/B/U-Throw. Save F-Throw for killing near the edge if you ever get him to a percent that he'll actually die at. Immediately follow up the throw with D-Special to pull out another banana (only if you don't already have two).
-Keep a banana in your hand whenever you're on the ground. You can use it for spacing (GT forward or backward), full retreat (dribble, essentially GT back throwing the banana down then regrabbing the banana), offense (GT forward, trip the opponent, get a free grab/D-Smash/F-Tilt), escaping chain grabs (Z-drop while you're being sent out of the grab, the Dedede will trip or see the banana and stop)...there's just so many uses for this one item.
EDIT: How convenient that the video in the above post has a good Diddy in it:
http://www.youtube.com/watch?v=-AUca1b8n4A&
Take note of how much glide-tossing there is.
That's all I can think of at 3 in the morning. Hope it helped.
Also if any better Pika's see this, feel free to correct anything. Just speaking from what I've seen, not what I've played.