This matchup is stupid, and after all that i've read I can't imagine I'm going to say something that hasn't been posted, but from my overall matchup experience I'll try and convey my thoughts.
Overview-
K so normally you'd look at this matchup and probably think "pikachu ***** this penguin", although this matchup can be **** for pika it doesn't really have to be. There are a lot of things d3 can do to pikachu, and If you're not informed you'll be falling for them.
K so lets look at the specs.
DDD
Can you cg him? No
Are you heavier? Yeah
Easy to edgeguard? Average
Pikachu
Can you cg him? Yes
Are you heavier? No
Easy to edgeguard? Below Average
Alright lets address the first thing. Pikachu is a very quick character, and avoiding the grab with dedede can be very difficult. With the help of platforms, good timing, and abuse of your range avoiding grabs will be made much easier. I reccomend banning FD in this matchup, as it is your worst neutral in this matchup. K now that FD is out of the way we have 3 neutrals *or if you're lucky 2*. SV is somewhat win/lose. The win being you have a floating platform to camp on, and escape his grab tricks with. The lose being, the stage is kind of large, and there is a lot of room for him to cg you. BF may just be the best choice for neutrals. The small space allows you to easily pressure the rat *what you want*, It also allows you to camp him very well with the 3 platforms. Ultimately the stage choice comes down to where you're most comfortable, but thse 2 are your best choices for neutrals.
Now when the match starts, the rat is going to be fishing for grabs. Be weary, and do not do anything too laggy, or too risky. The cg last from 0 to around 70 and leads into nair, or if he reads you right fsmash/dsmash. Ways to avoid being grabbed by pikachu are: grabbing him before he grabs you. Your grab range is ****ing huge, his is tiny, think about it. If he's dash grabbing you, your standard grab beats it. Your pivot grab beats out just about anything he can do on the ground as well. Another good choice to do when trying to avoid grabs is: Inhale. Inhale acts as a grab, and also beats his grab. If they see it coming and try and spot dodge or shield they still lose, and take a good 10. This however can be risky, if the pikachu decides to fsmash/dsmash. Lastly just camping works. Space your ftilts and waddles effectively to beat out his shallow ground game.
Now that we're done with the ground lets move onto in the air. This is where pikachu will be doing the rest of his damage after/if he manages to cg you. His QAC to nair, uair, fair, and bair mixups are all extremely good against d3. Our large mass just makes us easy targets for his fair, and bair. If you get caught in one of these aerials, just SDI out and try and punish his landing with bair/inhale. If you're getting juggled by his uairs, just try your best to get on the ground and to continue beating him there. Now this may be coming off like d3 is just completely ****ed in the air, this is not true. D3 does have his super, amazing, spectacular, fantastic, back air. Your back air beats everything he can do in the air. There is nothing he has that can stop your back air from ****** him, you just have to be smart about using it.
Lastly, the thing that frustrates me the most about this matchup OFF STAGE ! K so pikachu gives the penguin a lot of trouble offstage. He has just the tools needed to keep your fatass from making it onto the stage. He has a jab that can force your up+b if you do not sweet spot the ledge. He has a tjolt that can be angled, and used to make you lose a jump. He has QAC, that can make you lose a jump. Nair and Dair, can knock you even farther back off the stage, and he can go out pretty far *stupid up+b*. If pikachu forces you to up+b you're pretty ****ed. His most powerful attack can be used to send you way off the stage, and even provide the rodent with easy kills on d3. If sweetspotted thunder can kill d3 as early as 110. What I try to do when recovering in this matchup, is throw out bairs while floating backwards. By doing this you have an attack that is covering you, so you wont be getting hit by tjolts, or any attack he's doing on stage. Now if pikachu is off the stage there are a few things you can do to him. If he's charging a skullbash, you know exactly what to do. Hop off the level and throw some B's on that *****. Bair beats skullbash easy, and isn't that hard to time *if you do manage to **** this up, be prepared to tech*. If he's going to QA, this can be a mindgame. If you think he's going to land on the level, hop up and grab him. If you think he's going to go for the ledge, just maintain youre position. Another thing you can do if he just land right in front of you while you're on the ledge is dededecide. Edgeguarding pikachu can be pretty difficult, but If you know what you're doing you can put some damage on him.
Another note about the match. PIkachu is a very campy character, and If you do not know how to deal with camping, or if you're one of those people that panics when getting camped this may be a problem. The only thing he can really camp you with are tjolts, and these are beaten by our: ftilt, bair, waddles, and dtilt. Use these moves to cancel the tjolts, while slowly cornering pikachu. Once you have him cornered, it's a matter of what you think he will do/what the best option is. Don't panic if he manages to get away from you when you corner him. Just repeat the process until you get some damage on him. If you're ahead in percent, don't approach him *obvious* let him waste time camping, and just don't get hit by the tjolts.
~Matchup Note: If you are hit with a usmash around 100%, your first reaction could be your last. Using dair to try and momentum cancel after you DI can cost you your stock, as pikachu can read and thunder you while you're still near the blast zone. Keep this in mind, and try to maneuver away from the pikachu, and If you're hit be sure to DI!!!!!!!!!
All in all this matchup isn't so bad for d3. Just be patient and know how and when to punish the pikachu, and you'll be fine.
60-40 Pika, potential 55-45 PIka