Nair is your lifeline in this match-up. Nair will cut through t-jolt like a hot knife through butter, and go right on to slice up Pikachu as well. Any time Pika goes for a SH t-jolt, just nair, and you'll have a massive opening. Pikachu also leaves huge openings when doing full jumped t-jolt for Plasma Whip. If you predict a full jumped t-jolt, use Plasma Whip (you will hit if you space correctly, then just continue past the t-jolt. The result is that ZSS shuts down Pikachu's spam game, which means the rat has to rely on offense. This isn't such a bad thing for Pikachu, since he's no slouch on offense, but it means that ZSS has the defensive advantage, which is really useful.
Regardless, don't get hit. Pikachu is one of those rare characters that can actualy do some combos, and he'll uair u all dai if you let him. Get out of there ASAP, and continue to work the defensive angle until you can start a dazlock or land a dsmash.
Pikachus love dsmash, and with good reason. The move is difficult to punish on block and does massive damage. It is imperative in this match-up to learn to SDI up on dsmash. Otherwise you will take a ton of damage, and you will be perfectly set up for a Thunder. (You can usually airdodge or Flip Jump out of this combo, but it's better safe than sorry.)
SAVE YOUR KO MOVES. Pikachu is almost impossible to edgeguard, and won't be dieing until around 150-170% from anything if he's DIing and momentum canceling at all. Futhermore, his recovery is absolutely incredible. You can't just go out and gimp every time because he'll be spamming t-jolt, and he'll generally be at a good angle for them to hit. If you find an opening, go for it, but don't expose yourself too much, because Pikachu is perfectly suited to turn the tables here. For the most part, against good Pikas, I wait for them to recover, because he has an option for any edgeguard you can do. If you try to go offstage, he'll QAC onstage. If you stay onstage, he'll go for the ledge. I prefer my opponents on the ledge and me onstage rather than the other way around, so I usually don't edgeguard Pikas too aggressively.
On your recovery, be patient about getting on the stage. Pikachu will always cover the ledge with a Thunder if he has the time to set one up. It's best to wait the thunder out and just tether the ledge when it's done. Once you've done that, it's really a guessing game as to how he'll defend the stage. Guess right and you'll be back on stage. If you can manipulate him into trying to gimp you, that's even better, because your uair will be able to protect you. But be careful, because t-jolt eats ledgestalling for breakfast.
Overall, I think ZSS has a slight advantage simply because she can play defensive, and Pikachu can't, meaning she's got more options in a neutral setting. Basically, ZSS should get that initial advantage more often than Pikachu, and that will tip a match-up that's otherwise fairly balanced.
55:45 ZSS. 60:40 on a good day.