I'm late, sorry. Was driving across country. Missed KK's call.
Pika vs. jiggs is annoying because jiggs is annoying. Pika doesn't have much useful against puff. Puff is tooo light so you can't combo her and she's impossible to gimp. So you have to rack up 60+ damage from single aerials (or uair to uair, ooo 6% combo) and then try to get a usmash in (gl usmashing a competant puff with pika), or rack 90% and try to bait an opportunity to fsmash her (more likely), or rack like 110% until one of your aerials will finally kill. Main problem with the racking of damage on jiggs is her stuff beats yours. Bair goes through like everything, and her fair is easier to set up than most of what pika has. Pika's aerials are kinda awkward against jiggs. Uair hits but either just leads to another uair, nair at low percents, or sometimes nothing at all. And uair's damage is terrible. For pika's nair you have to get relatively close and it's beaten out by jigg's aerials anyways. Fair can work but if jiggs just DIs up and away she takes like 2% damage and pika can't really follow with much most often, while jiggs usually can get away or sometimes retaliate with an aerial before pika's done lagging. Pika's bair can be a nice surprise against jiggs, but it's hella risky. Jiggs punishes risky with rests, grab followups, and fsmash most commonly, so ew. Dair beats jiggs' aerials, but it takes setup time and she can usually just avoid it or shield it.
In ground game vs jiggs, you have to watch for her grabs, fsmash, and utilt primarily. Utilt to rest works wonders, and it's not too hard for jiggs to get close to pika with aerials (just watch out for usmash out of shield, but usually pika will have too much shield stun to do this as long as you space well). Pretty much anything pika misses on the ground is punishable by either bair/fair out of shield or utilt rest.
But yeah it's not all bad. Usmash kills jiggs on most stages at like 60%. So if you can get one in earlyish, it can create a quick stock advantage. It's kinda hard to set it up, jiggs knows you have it and can space aerials so you can't rush in range of it. Nair can lead to it at low percent, but it's pretty DI dependant and you have to hit with the end of the nair and they have to be at just the right percent and etc. but it can work occsionally. You can also uair spike them into the ground (if you get below jiggs) and hope they either standing tech, techroll towards you, or don't tech and then try to follow their eventual getup, at which point you read them and usmash.
Also, other than aerial camping, utilt rest, and occasional techchasing pika's terrible techroll, jiggs can't do much to pika. Pika should be able to get around pretty much all of jigg's edgeguards. Though you can't really edgeguard her either. Jolt works ok against jiggs, it just doesn't work too well as an approach because of jiggs' oos options.
It boils down to jiggs having more priority than pika, and none of pika's usual stuff works. You both just throw moves at eachother until one kills, as with decent DI neither character has significant combos on eachother (outside of the occasional aerial->usmash or utilt/aerial->rest). And jiggs just happens to be better at the single-hit-at-a-time thing than pika.