I'm a Pika main who just happens to have a friend that mains Corrin, so I'll try to add my two cents in here if there is no problem. Corrin outspaces Pika just the way Marth, Lucina and Cloud do but Pikachu outspeeds Corrin way to fast, we don't even need to QA actually to get in if we wanted to as our ground game is better and Pika can aproach, shield and keep aproaching, also we can outcamp Corrin with jolts as a full hop jolt can neutraliza a fully charged dragon shot and the bite has some quite lag afterwards making up for a grab or JCUS if Corrin is at kill % Offstage is not easy for Corrin either as Pikachu has a lot of ways to gimp Corrin, you might thing Corrin's Up-B is pretty safe because of it's huge hitbox but well... Diddy's tend to think the same about the monkey flips and tether recoverys think they are fast enough. well...
https://youtu.be/auR4b2kxQt8?t=167
https://youtu.be/6R_cMPjU6X4?t=50
Huge hitbox doesn't mean safety ( I have one of this on my friend but the match took like 4 minutes and I don't have a capture card so I can't showcase Corrin right now) BTW, this is a true combo if the first thunder hits.
Also you might not want to go offstage while edgeguarding as he can mix up his QA and even thunder spike you if you get too greedy. Also I0ve survived a couple of down air spikes here and there, so don't go for that.
What Corrin can do however is to try to float just outside of Pikas space by staying in the air and mixing it up with Side-B. In the air Pika needs to commit to an option and there's when you can connect a fair or a nair to do your stuff. Also instant Dragon Lunge can punish Pikachu really hard is it comes out really quickly, it kills and allows you to retreat. Don't throw F-smash on neutral as we can get a lot from a grab or nair.
On the edgeguarde the best and safest option in my opinion is getting the snapback on the ledge, if Pikachu has already burned his second jump you can nair near the ledge (offstage) to try to get an stage spike or something like that (beware that if he has a second jump he could double jump thunder from the deep and spike you).
For killin up air might do the work and your hard read smashes. Dragon lunge works really well too and obviously a grab casue Pikachu is really light
tl;tr Keep pika in the air, avoid ground game and camping. Side B punishes hard, don't mess with him offstage.
55:45 on Pika's favor in my opinion