Nid, I love how you're at least honest.
Even if you seem to throw out ANY theorycraft (some of it's legit, and most of my gameplay actually comes from theorycraft =\ I back up my theorycraft with actual gameplay... haha)
But I try to be careful with my theorycraft. I don't assume perfection in it, and I make sure to recognize both players options in given scenarios - and I add the human factor of prediction and mistakes, to some degree. (Add to much, and your theorycraft is rendered useless to be honest... )
But A2 is just pure theorycraft, and ignores any of the other sides' options and completely ignores the human elemant.
You know, I actually play this game irl, even if it doesn't seem like it.
I wouldn't confidently state that Mario has trouble against Jiggs without actually playing that matchup. Your experiences seem really shoddy, when you're limiting yourself to a character you clearly have much less experience with, when someone like vato has infinitely more experience at spacing and mixups than you do in a specific matchup.
And I'm pretty aware of the human element. Reaction time, accounted for threats, and mixups are all part of my equation.
Mario's spacing can only do so much to wall Jiggs, and her damage output is slightly better than Mario's. When in neutral stance, Mario can either throw a fireball, or attempt to wall with a tilt/B-air. Tilts are unfavorable if they trade (may or may not happen depending on spacing), B-air is good except it's whiff punishable on reaction and can be crouch baited if done rising.
If Mario is facing forward, he has nothing to punish a spaced attempt from Jiggs. He will block. N-air and U-air generally will not reach Jiggs unless she tries a crossover.
If facing backwards, B-air is an option, although powershielding is sometimes necessary to punish good retreat spacing and even then depending on the situation, it might not be guaranteed. Mario however has MUCH less air mobility than Jiggs, which means Jiggs clearly has the baiting advantage, and baiting is a COMPLETELY legitimate strategy if you have good mobility and zoning threats like Jiggs does.
If Mario B-airs, either he spaces it just right and it hits, or if Jiggs baits it, then he gets punished and potentially comboed, which immediately makes him risk losing the lead. In a direct clash with B-air and Pound, Pound comes out on top or iirc trades favorably in terms of damage.
This is the way the matchup goes for me when I play it. Honestly I don't know what you're doing wrong. It just sounds like your spacing sucks from what you're telling me. Jiggs can legitimately bait Mario relatively easily into making a mistake that will get her the lead, and then the matchup goes in her favor. Even if Mario tries to not get baited, his shield only lasts so long and doesn't cover his head that well. Plus attacking Jiggs's shield is rather unsafe for Mario unless he perfectly spaces (implied commitment to fastfall spacing), since Jiggs has a better shieldgrab and F-air than he does.