I should point out as well, Jiggs's DA hits you at a pretty low angle and comes out pretty fast. Her grab is also MUCH better than yours, so it's not like she can't do anything to you on the ground.
And what options does Mario have to save himself from getting gimped in this matchup that Jiggs herself can't bait and counter? She's one of the better edgeguarders in this game, and like Metaknight has multiple jumps and solid area control. If she chases you down far offstage, what are you going to do?
Mario does beat her slightly due to being heavier and having more KO options (you can Jab cancel to a lot of dumb stuff on Jiggs, like Jab -> turnaround U-smash lol), but she definitely has tools she can use to counter a lot of Mario's spacing strategies, and even though Mario may have good combos, he doesn't have particularly many options for getting out of her B-air strings either. Yes you can trade hits in this matchup, but that is more or less mandatory if your opponent has really good spacing. Jiggs doesn't really care which direction she's facing you as long as she can hit you offstage. Mario however has his options briefly limited when he's hit and forced to face forwards.