little mac beats jigglypuff. Any smart little mac wouldn't go near the edge and get backthrown. They'll keep control of center stage and make you fight them there from the start. You'll almost immediately lose percent lead against him do to moves like dash attack, ftilt, jab, etc. dealing a lot of damage and having massive priority. You'll almost always be playing from behind since he gets % much easier than you, and his ground attacks have more priority than Jigglypuff's aerials. Playing from behind against little mac is awful, because all he has to do it wait center stage and beat practically all your approaches.
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OddCrow
I went to a regional tournament where several of the higher level players from Oregon and Washington played. Gimpyfish, who you probably know since you're an older member of smashboards, won that tournament using Little Mac. I played at least 30-40 matches against him during the day, and I had only won about one match where I was puff and he was mac out of like 8. I won more than I lost using Ness, and even ROB, due to their disjointed hitboxes and access to ranged attacks which forces mac to approach no matter what. That's the main weakness of puff in the matchup, Little Mac has no reason to approach you when he's ahead, and it's so easy for him to get ahead as long as he doesn't let himself get gimped.