I have played both sides of the puff matchup and it's not hard at all for jigglypuff to get in on marth. she's a thousand times more flexible and viable when she's moving in, and she literally just has to come close to marth and kinda sit there until marth attacks, leaving him open for a bair, fair, and anything that follows up on it. vs a smart jiggs with marth, she's not going to run into marth's sword and she WILL wait until there's an opening, which really isn't as hard as it sounds, and the only hits marth will get off will be a random fair here and there because of seriously outpredicting the jiggs player's movements or just as a mistake by jiggs...unfortunately, it really doesn't lead up on anything, so it doesn't matter. when jiggs gets into marth, she has plenty of options and can combo him. as well, when marth is off the edge, jiggs edgeguards him way better than any other character and edgeguarding jiggs is a pain with marth because you don't want to risk going off and accidentally getting pounded, losing a stock. and if marth tries approaching jiggs to try and stop her from sitting there and waiting, she always seems to avoid it and punish marth for doing so, whereas jiggs really isn't punished by approaching marth as much.
it's really not a pretty matchup for marth. it would seem like a matchup that marth dominates in theory simply because marth's spacing is theoretically superior with his range, but in practice, the matchup is roughly even as jiggs is much more flexible and navigates around with much more control than marth.
and as m2k has mentioned before, marth needs some critical elements of his game to win, them being grabs, gimps, early percent combos, and edgeguards. marth can't gimp jiggs, can't edgeguard her, can't combo her, and normally doesn't (and shouldn't be) getting grabs off her, so I don't know why people think a sword is all marth needs to outspace jiggs when jiggs has superior spacing to any other character.
