More contribution, less ridiculousness, please.
Assuming it's 06 where the Puffs run into a camping Marth anyways, sword range is irrelevant, a great many characters out range Puff with even more power/less lag then Marth (I don't mean super laggy forward smash, I mean like Sheik's b-air). Jigglypuff has no problem getting inside your range, no matter who you are. I think we can agree it's not a good way to anaylze the match up to say something like "use your sword and range, just never get hit". I think it comes down to how efficiently Puff kills Marth vs. how inefficiently he kills her.
Because of her unique mechanics whereby you can't really combo her or punish her mistakes for more then maybe 25%, but usually a lot less, she can make mistakes at a ratio of like 3:1 on yours; she might die off the side/top at like 90-100% if you get lucky (if you're not fox, for him it's earlier) but that's all 'single' hits. So that's maybe 7+ hits you need there assuming 13-15% a hit (average aerial) and assuming you can get the gamble right in the end with a f-smash (restable btw if you miss). It's almost like Brawl for her, you shouldn't approach but even if you do, you're not gonna get punished for more then 1 hit, maybe 2 anyways.
So you can try approaching and preventing her from setting up. Yeah try to approach for 15-25% but what if you mess up your approach or she calls you for what you're doing? Then you get *****, the bigger the payout you're going for the more likely you are to die if you mess up. Is it really worth it to have to gamble 15-25% damage repeatedly if you'll take a combo back for it? or a rest? You have to be right at least 6 times if you go for a big punish each time, more for something that'll do less %, but she only needs to get it right once or twice for you to die.
On the flip side, she's not gambling to kill you. Her game is so safe, especially since she can match your range in terms of how far from wherever she's standing she can throw a b-air out and land back where she was. It's just camping space back, weaving around your sword with b-air camping, shielding whatever you do to a free b-air while hoping you try to grab or f-smash or whatever. If you whiff a grab you die as Marth. She's just hoping for what Marth is; that the person sitting beside you will a mistake first so they can punish. Except she can get decent punishes. It's not just random single hits, she can keep following up and tech chase or combo.
TLDR; how the **** does Marth actually kill Puff without putting himself in danger of dying? She kills him at a steady pace (at least, I'm being generous and assuming Marth will never get rested, because if he does and goes down like that he's never coming back) with b-airs/edge guards compared to his erratic pace from constant gambles and guesses. This inefficiency on Marth's part makes me think it favours Puff/makes the Marth players go Fox.