Yeah...in this Matchup wifi isn't really a reliable source for information since neither character can play anywhere near as well online compared to offline.
My brother already posted, and i agree with everything he said. However, i'm gonna say say some things from the pikachu's perspective.
ZSS is a ***** to fight. Both characters basically HAVE to put the game where they want it, and since both characters have pretty good all-around mobility, this game will be shifting a lot if the skill between the players is even.
In the air, both players have an advantage depending on where they are according to their opponent. If pikachu is above ZSS, ZSS has the advantage because of her amazing uair. However, if you aren't careful pikachu can FF a nair or dair and put the position back to neutral. If pikachu is in front of ZSS (both facing each other) pikachu has the advantage. ZSS fair isn't very applicable in this MU unless comboing out of D-smash. Pikachu can either uair you (hitbox is quite large in front), fair (if you are close to the ground this will lead to some problems for the ZSS) or move towards you and nair (decent knockback and damage).
If pikachu is behind ZSS, we lose...hard. The only thing we can MAYBE do to escape is to FF airdodge to get back to the ground, in which case the ZSS should fall with a FF plasma whip (i think that is forward B).
If pikachu is below ZSS pikachu can have fun. Uair juggling and air chasing are what pikachu does best. Upon landing after the air ****, it is your best bet to bair. If we shield, we can't punish unless you didn't space correctly. If you did space correctly, the only thing we can MAYBE do is to FH a T-jolt, but that is all prediction and habits. If we know you are gonna use a ground attack, the T-jolt will work. However, if you know we are going to T-jolt, you can run and uair us, putting the pikachu back in an unfortunate position. Most of ZSS aerials are NOT safe on pikachu's shield. Against basically all of them except a tipped bair, we can run and grab or u-smash you. I have been able to punish Nick Riddle against every other thing. The only things that pikachu cannot punish are tipped plasma whip (assuming we don't run or anything to get to the sourspot or to PShield the weak hit) and tipped bair, so GL trying to keep us away
On the ground the fight is interesting. Both characters are fast and have a pretty good ground game, but pikachu has a major advantage. Being able to use our grabs. ZSS grab game is pretty non-existant against pikachu since we can always sidestep it (like most characters) and punish you with either our CG, the uair-footstool-QA combo, or a quick little FF fair-grab-pummel x2-d-throw-nair/u-tilt (assuming you aren't higher than 50%). So it is best for ZSS not to even try any grabs against pikachu. The reward DEFINITELY isn't worth the risk. Also, on the ground, pikachu is faster than ZSS. Yes you have more range, but with good Pshielding that can be eliminated real quick.
Killing potential is there for both chracters. Pikachu's kill moves are (in order) thunder 55% (blue hitbox), F-smash 90%, U-smash 120%, D-smash 140%(assuming you don't DI it) thunder 160%(bolt), U-tilt/F-tilt 165%. However, u-tilt and u-smash can be comboed into thunder at around 90% to kill, so be careful or you will get killed early.
Pikachu shouldn't be surviving longer than 130% against ZSS. If you hit us with a D-smash at 110%, we are dead if you bair us. However, since that is one of your safest aerials, you might have it diminished, as well as plasma whip. Try to keep at least one of those moves (plasma whip, bair, uair) fresh so you can kill. All of them kill pikachu around 130% fresh, even with good DI + momentum cancel.
Edgeguarding can help pikachu get a quick kill on ZSS. WIth the help of T-jolts and the lasting hitboxes of nair/dair, pikachu is a threat to ZSS when she is off the level. If you aren't careful, the T-jolt can gimp your jump, allowing the pikachu to grab the ledge which means a ZSS death. Also, pikachu can thunderguard (Wavebouncing a thunder off the level), which can basically stall time and prevent you from tethering to the ledge. If you are going to die from that, ZSS should get hit by it since it will bring them up. However, the pikachu isn't always trying to get a kill from edgeguarding. With teh extra %, we can set up for our kills or just knock you off the level against with one of our aerials.
Adversely, pikachu CAN NOT be edgeguarded by ZSS unless you read us 100% perfectly. The only thing that you guys can punish is our Skull bash, but pikachu won't need to because of our QA (one of the best recovering up-b's in the game). Hell, if you come off the level, the pikachu can hit you with a nair or uair to get you in a bad position, and the pikachu can QA back to safety.
So, i'm gonna break down the position of the characters and who has the advantage.
P air vs Z air = Z+ (you have more advantageous positions)
P air vs Z ground = P+
P ground vs Z air = even
P ground vs Z ground = even
P off the ledge vs Z edgeguarding = even (pikachu won't be edgeguarded)
P edgeguarding vs Z off the ledge = P+ (ZSS can be edgeguarded. Not well, but it can work)
As a ZSS player, you HAVE to get pikachu in the air in order to win. If you stay on the ground, you are going to lose if the pikachu is on the same skill level as you. Pikachu's damage racking will most likely come from T-jolts, CGs, and quick comboes. Pikachu wants the ZSS to stay on the ground while we can be SHing to produce quick damage/comboes with fair and uair. Basically, this match will come down to who can predict the opponent better and pick up their habits.
50:50 or 55:45 pikachu. Nothing else.