What are pikachu's mixups?
A mixup is literally just doing something other than what is considered "standard" - as an example, Nairo will fsmash after dsmash as ZSS [standard], but he'll also do Boost Kick, jump to where they pop up and boost kick, go out for a uair, go for a bair, or even go for a down+B spike. These are ALL mixups because some of them require different vectoring/DI from what's usual.
As a result, just isolating mixups is hard, but one very basic mixup is to do a fastfall fair and then either dash behind them and usmash [expecting a roll back], grab [expecting shield], wait a moment [looking for a spotdodge], or fsmash behind you [expecting a roll behind you]. These are all mixups because fastfall fair rarely true combos into grab [definitely can't be ocmbo'd into usmash consistently].
Another easy to explain mixup is bair offstage - you can do it and try to stage spike them, OR fastfall it so they go lower and hope they try to tech, which will result in an airdodge and death.
I posted this in the MU thread, maybe this was a better spot. I have a lot of trouble with my friends Luigi, is it a bad MU or am just bad? I can beat him and just do a lot better in general with my other characters (Kirby, Greninja, Charizard, and I even managed to two stock him my 2nd game with Roy.) but with Pikachu I struggle to even keep it close. He seems to have an answer for everything I throw out. Just to list a few things he gives me trouble with.
-Hard to combo because of nair
-Fear of Shield grabs and U-Smash makes approaching from the air risky
-Fireball can cancel out T-Bolt
-Smashes come out fast, hard to punish.
From what I've seen, uair and well-space fair can beat his nair, so you have to have very tight spacing to effectively combo him. Dair from above should beat his nair as well [I was told by a Sheik main that our dair beats her uair...], although that's impractical for combos.
For approaching from the air, I THINK SH dair has good pushback, but you have to commit immediately for that, so it's far from an effective aerial approach when deciding while already in the air. You can do crossovers and also attempt to use fair, but spacing fair is very difficult [I think we need someone to confirm if it can actually be spaced around shieldgrabs... I think it can but it's hard at best].
If he's doing fireball to clank t-jolt, you're probably doing something right. One mixup is to do a fullhop or SH to try to bait a fireball, then QA into him in fireball endlag and go from there [though obviously you don't always want to do that or he'll space around it or not fireball and punish]. Also because of how t-jolt works I'm pretty sure they don't always clank [sometimes they go through each other].
His smashes just are not easy to punish, but fsmash if he just plain whiffs and you can react is good, since long-range fsmash on Luigi's shield is very hard to punish unless he powershields it [high pushback on fsmash + low traction on Luigi]. If he misses a usmash, ftilt or dtilt him for at least a bit of percent and a potential jab lock if he misses the tech at certain percents. On shield, I think you can either try to punish a spotdodge or roll, but otherwise it's probably safest to simply have a tactical retreat [go back to neutral and poke more].
Hope some of this helped.