I personally believe the match up is 50-50. Pikachu has a better ground game but Peach has a better air game
Pikachu has a variety of ways to build up damage. He can Down Smash you, grab you ---> constant Up Tilt ---> various aerials or just spam you to death with Thunder Jolts combined with Quick Attack or just simply camping with them
It is essential you learn how to Perfect Sheild Pikachu's Thunder Jolts as it will make this match up much easier. Pikachu has reasonably significant lag after using them. By perfect sheilding T Jolts, you can punish a Pikachu who has poorly spaced his T Jolts. Floating works surprisingly well in this match up as it gives you the ease to move backwards and forwards in the air to avoid his T Jolts. Pikachus T Jolts are fired digonally down the way he is facing so if he's going to hit you in the air, he's going to have to get pretty close/slightly higher than you. Make good use of your 2nd jump in this match as a spacing/dodging tool and don't be afraid to get in close and take advantage of the vunerability Pikachu has after using his T Jolts.
Make use of Dairs in this match but be aware of Pikachus Up Tilt - it comes out fast and if you time your Dairs wrong, you'll get a face full of Up Tilt. If Pikachu attempts to continually Up Tilt you, try Floating out the way and if you think Pikachu will jump up to try and hit you with an aerial, get a Nair in there. Turnips are helpful in this match as well. Glide Tossing works but Turnips are cancelled out by Pikachu's T Jolts. Time your pulls well because Pikachu is quick and can punish you if he's close by with a Dash attack (or worse, an Up Smash). I personally like to save my Turnips and throw them in the air to counter a jumping T Jolt spamming Pikachu
Pikachu has decent aerials. Uair has low knockback and will mainly be used for juggling. Bair is a damage builder - if you get stuck in this, DI upwards out of it. Dair has a very very deceptive hitbox and as an additional hit when landing. If Pikachu uses this above you, get ready for on Up Tilt. If he SH's it, Sheild it/Float out of the way and be prepared to punish his landing lag. Pikachus Fair comes out fast, has reasonable range and has multi hits. If Pikachu SH spams this, counter it with by jumping back and coming down with your Fair which has more range (this is difficult as Pikachus Fair has more range than it would first appear). If Pikachu uses fast falled Fairs, depending on where he his, counter it with an Up/F Tilt. If that doesn't look like it'll work, sheild it and roll out the way (this is by far the safest option). Getting hit with Pikachus Fair can lead into some nasty moves such as following into an Up Tilt/Down Smash. You can punish poorly spaced Fairs by walking backwards and using F Smash as well. Remember, if you're close enough, Nair will outprioritze/trade hits with Pikachus aerials
Pikachu has a variety of kill methods and his killing power is better/more flexible than Peachs. His main ones are his F Smash, Up Smash, Down Smash and Thunder. If you get stuck in his Down Smash, Smash DI up out of it (be prepared to tech on the floor). Never EVER try to hit Pikachu out of his D Smash with an aerial until he's finished using it because you WILL get hit. If you sheild the D Smash, wait until you are out of hitting range and punish Pikachu with an F Smash. Be aware of clever Thunder use! If Pikachu knocks you up into the air and you're at a high percent, don't get tricked as this is a very easy kill for Pikachu. Pikachu can wait for you to airdodge and then punish or run over to where he thinks you'll move in the air and punish you that way. Or if he's feeling really technical, he can use QA to dart about/catch up with you and use Thunder. If you get hit by Pikachus F Smash, your best bet is to DI diagonally upwards as DI'ing straight up risks a ceiling kill
Watch out for Pikachus Quick Attack. Not only is this good recovery, Pikachu can use this to perform the QAC and zoom around the place to enhance Thunder Jolt spamming (jump up and B reversal), rack up damage and jump out with aerials. You can knock Pikachu out of this aerials/smashes with nearly anything in your moveset. Actually doing this though is difficult
Pikachu has a more versatile recovery than Peach and whilst it's difficult to gimp, it's not impossible. Turnips are useless for gimping unless you're trying to get some extra damage on Pikachu. Watch how your opponent recovers and take advantage of it. Nairing him out of the Quick Attack isn't easy but it can be done. Be ready to grab onto the ledge for an edgeguard if you think Pikachu is going for the ledge
Toad is very useful in this match up, paticularly against aggressive Thunder use (never use it high in the air though if Pikachu is trying to kill you off the top) and close T Jolt spamming. Make good use of him but don't spam him (as always)
So far, after taking everything into account, I think the match up is 50-50. Both characters can rack damage up against each other, both have a decent projectile (Peachs is argueably better in this match) and both have reasonable KO ability (Pikachus is better when he lands them correctly though. Peach's Fair isn't a very good damage builder in this match up so it will be kept fairly fresh which is a bonus)
