Depends on how they grab you, too. I'll just throw a ton of info at you and hope some of it helps depending on your situation, haha
First off, single jabbing is good, use it! Pika's is a little worse than most characters above him on the tier list, but it's still effective to counter grabs at times. They'll be trying to grab and get interrupted by your quick jab, and you can usually punish them before they realize that their grab got interrupted in the first place. Jab is useful, too, because of how hard it is to punish for more characters. Pikachu's grab is a bit laggier than others though, so use it sparingly.
If your concern is being shield grabbed, then work on your spacing/cross-ups. Mostly cross-ups, since Pikachu's range sucks too much to space outside of grab range usually. Land behind their shield and they can't grab you (unless they do something proactive). If you land on front of their shield, make sure you're ready to do something fast enough not to get grabbed.
None of your aerials are safe to land on front of your opponents shield with, BUT an L-canceled nair only has 7 frames of lag and grab comes out on the 7th frame for most characters, so you can bank on your opponent not being frame perfect a lot and can usually get a jab or spotdodge before the grab. Do your best not to be in this situation with any other aerial. This info might not be super reliable since I don't play Pika, so axe/n64 lemme know, but just loosely consider this
Remember,
baiting is a great way to get free hits in on less experienced players and counter people who fish for grabs. One huge technique is jumping at your opponent and doing absolutely nothing until you land - aka empty approaching. They'll be expecting an attack, so you'll have not only the frame advantage but the reaction advantage on them, you just can just land on front of them and grab them out of their shield or something. Faster characters like Pikachu should abuse this a lot, I play puff and it still works all the time (since they expect bairs lols) so that just shows how effective it can be. If you're advanced enough to do this, auto-canceling your approaches is also a fantastic tool, it's just like empty approaching except you cover more options because you
do throw out an attack AND you can still interrupt them. In general: be spontaneous! If you get predictable or they read your baits, you'll continue to get grabbed.
If they just grab you a lot, consider the following:
- Pikachu is really agile, don't stand still much at all. Try to have pretty though-out and safe movement and just be smart about how you approach/dashdance
- Be safe with attacks. Don't throw out random smashes, like don't get too greedy for dash-in usmash and such.
- Change up how you tech. Being predictable = getting punished.
- Try not to roll too much, you'll find yourself getting punished a lot unless you do it right.
In general, you can choose to take either a defensive or offensive approach. For defensive, try to get good at your dashdance game, and get use to simply dashing away from your opponent when they approach and dashing right back to punish them if they do something, or just keep backing up if they do nothing and reset the situation and repeat. The offensive method is using mix-ups, quick and rapid attacks to overwhelm your opponent and make them not want to grab because of how unsafe it is. Less experienced players will usually respond with bad rolling, where you can read their rolls and get follow-ups. Mixing up between these two styles will help your game a lot, but they both take a lot of experience to master. You'll have to do some research or ask questions to get more info on this for your character.
If I learned one thing about Pikachu from watching Axe, it's that cross-up nair to up-air is the mo****ing godlike so you better practice the **** out of it if you want to win. (This implies knowing your follow-ups, especially versus those fastfallers. You can get a anywhere between 50% and a kill by just getting a good up-air on a fastfaller if you can convert it.)
Most of this was not written in regards to top level play, but to be helpful to someone who's still in the process of getting better at the game. Also, didn't really look over what I just wrote so sorry for redundancy, etc.
Feel free to ask any question or clarification lol. Hope this helps.