Funny story, I would regularly in tournament play pikachu vs. 25 of the 26 selectable characters. I switched to young link when fighting jiggs. I hate pika vs. jiggs.
Having said that, it's still not super horrible. Most of what's been said already is true. Try to stay below jiggs if you can, she doesn't have much to do to you from there, and you can come up with uairs and nairs relatively safely, while threatening to usmash if she tries to approach you from above. Practice usmash out of shield and cc usmash. Though you probably won't have much opportunity to connect with either, as jiggses like to poke you with aerials and not actually approach until one hits, it's still pretty useful if she tries to like nair your shield or actually approach you. As tdk said, when you get hit off the stage, DI away (as long as it won't initially kill you) if the jiggs jumps to chase you off the edge. Really, you want her to try to approach you as you're recovering, and then zip by her with your upB and attempt to edgeguard her. Otherwise, sweetspotting is useful if you can position for it, as jiggs doesn't have much to intercet that well. If you're edgeguarding jiggs, just throw jolts out if possible and try to repeatedly uair spiker her if she runs out of jumps. Be weary of approaching her off the edge though, jiggs' aerial mobility makes her relatively good at punishing any approaches, even if she's recoverying at the time.
While you're both on the stage, things mostly turn into quick hits. Jiggs will often throw out aerials around ou prevent your approaches and rack up damage. You need to try to get down the timing and jump in to punish between her attacks. Trading hits with jiggs is ok, just difficult to do, as her aerials beat yours usually. If you ever can get a usmash, by all means do. It kills jiggs at like 60-70% on most stages. Otherwise, tap at her with aerials until one kills.
Jolt is relatively safe to make her come to you. Most jiggs will just go over it, which is your opportunity to dash in and get under her, which is where you want to be.
Watch out for her fsmash, she'll occasionally try to bait an approach->fsmash punish. And yeah, it hurts.