He's probably the most "fundamentals" oriented character in the game. Patience, footsies, and looking for the right ins are the key to winning with him, and no one does it better. 90% of the Little Macs I play just dash attack all day long, and then get punished. If they get punished a lot, they start using side B instead (why?!). Maybe they'll charge neutral B mid-screen with some kind of hope that I run into the punch.
I think he gets picked because he's really fast and hits hard, and so you get the feeling of reward just by running around and pushing buttons if your opponent doesn't understand the mechanics well.
I think another reason for his low win ratio is him being a patience oriented character. I'm a lot more patient in actual play with Mac than I'd ever be in For Glory. I don't care about the win ratio that much and I don't got that much time. If you're using a cheap tactic, I'm never gonna wait it out in For Glory, I'm just gonna try to figure out how to get you to stop doing it.