*cracks fingers*
So I'ma talk about the closest ones first. Will be editing as i go
LUIGI
Quick synopsis: Both combo the hell out of each other. Luigi kills better on stage, Pikachu edgegaurds really easily. Pikachu can play VERY evasive and avoid luigi fairly easily. Luigi cannot punish good QAs. Careful approaches help a lot because of luigi's no traction. Pika controls the entire pace of the game
Ground-Ground: Pikachu DEFINITELY wins this part of the MU. Luigi NEEDS to powershield to get in, whereas we can just kinda...do what we want. If Luigi is on the ground just dtilt because he will slide back and not be able to punish. Any hit immediately causes pressure due to luigi NOT wanting to go offstage vs Pikachu. Also, d-tilt trip combos into QA which leads to everything else. Luigi's grab game is scary, but as long as you control space correctly you should be fine.
Ground-Air: Most rising aerials will hit luigi, such as rising fair or instant dair. Both of these lead into long long combos and luigi typically has to have aggressive ground move spacing in order to hit you like f-tilt or utilt. However, this is REALLY bad in the ground-ground game so if you frustrate them with that it'll be hard for them to hit you.
In terms of luigi in the air, anti air up-angled f-tilt is amazing vs luigi. It might trade with nair, but he will probably not be able to combo out of this regardless. Luigi is going to want to SH A LOT in this MU to overcome our ground game so we will have a lot of opportunity to anti-air him period. Also, if you expect a commit to a landing aerial, you can pivot backwards F-smash (OR HSB with customs on) to get big punishes and land kills.
Air:Air - Pikachu really wants to be under luigi if in the air because it is really easy to bait out nairs and air dodges. Being anywhere else and pika essentially just wants to get back to the ground because IF YOU COULDN'T TELL BEING ON THE GROUND IS AMAZING IN THIS MATCHUP
Off-stage: lelelelelelelelelelelele. Force him low with just your presence and they get scared. If they are already low, t-jolt and run off fair/bair. He can't really edgeguard you because QA 2gud. ALso if you get the bair FF multihit spike when he doesn't have a jump he dies regardless. yaaaay
Killing: Luigi obviously has more general kill potential, but it is pretty easy to push luigi off the stage. Also, pika forces characters to air dodge or throw out moves defensively so it's pretty easy to F-smash/U-smash that.
Yoshi:
Quick Synopsis: Yoshi can aggressively zone INCREDIBLY well and make it really hard for pikachu to get in. Pika's lack of range is obvious in this MU as we have no real way to poke out yoshi. Pikachu gets a lot when he does get a hit though, and he can get a lot of damage that way. Yoshi has no real way to push in if Pika has a lead, though, and Pika can run away from him really well. THis leads to a lot of punishes, %s, and maybe kill.
Ground-Ground: This situation never happens. Yoshi is essentially always in the air or jabbing after an autocancel Fair. Even so, if this was actually a thing, Pikachu probably wins. D-tilt > Jab and all yoshi really has on the ground is Jab and...dash grab. However, yoshi's grabs you can react to and punish REALLY hard. As in every MU, pika gets a lot off of his d-tilt so you can definitely capitalize on it. Yoshi's jab is obnoxiously good though, so be careful.
Air-Ground: When Pikachu is in the air he actually does really well. Fair puts a lot of pressure onto yoshi, as does dair. Getting into yoshi's space is essential in this MU so controling space with fair -> QA, it is a pretty reliable way to get in. Pikachu's amazing aerial mobility help get around eggs and being above YOshi isn't the end of the world because all he has is essentially uair which is pretty predictable. You can shoot T-jolts from certain spacing and begin a lot of your control which is how you beat Yoshi
When Yoshi is in the air and pika is in the ground, Pika essentially has to shield. You can't really beat out his fair unless you like SUPER PREEMPTIVE READ RUN U-SMASH pika can't really do anything. However, you can take up space after you shield fair which pushes yoshi back and typically forces him to retreat or jab (Which you can trade with). Typically in this situation I would rather be jumping than stuck on the ground
Air:Air - Pretty even. Pikachu gets a lot off of a single hit but Yoshi's hitboxes are more safe and you can also control space with T-jolts. Yoshi's aerials are better, but they don't really strong into each other well at all. Air dodging against yoshi's aerials is really good since they aren't really that fast in terms of cooldown. Pika's hitboxes are more lingering or very quick, so air dodging as yoshi doesn't do as much.