I actually love this matchup! Mostly because there are many tools that each can counter against. For example, the fireball can easily be avoided and cancelled, but get too close and Mario and either bair or uair you. Mario does beat a lot of puff's ground game, like the jab cancel smash shenanigans, but Jiggs can just float and stay off the ground. As soon as you get Mario off the stage, he's gimp city. A lot of Mario's I've played throw up bair walls, uair, or try to catch jiggs off guard with a timed smash. Ways I encounter this are: 1) just fly on the other side of him and bair him back. 2) bait a bair and as soon as Mario pulls in the legs, punish. 3) Attack towards the angle. Jigg's bair is janky enough that it would hit at a diagonal. To counter the uair, I just do a well-timed DACUS or do a well-timed dash attack . Most importantly, save your dash attack and fair untill they're in killing range (Mario is at like 90 on edge and dash is about the same)
How you play against Mario?
1) Bait and punish
2) throw off the edge and gimp
3) Stay off the ground
4) Mostly throw out bairs and pounds to rack up damage.
My Verdict: If it's just the characters, like just picking up, I think it's -1 (-.5 if there is one) Jiggs
If Jiggs knows what she's doing and so does the Mario, I think it's +0 even