The reason Puff is gradually placing lower on the tier list each time is simply because Puff is lacking something incredibly important, especially in the way Sm4sh has been made.
Jiggs can't deal with turtling, simply because it's actually easy to be really defensive in this game. And because of Puff's limitations gameplay-wise, it's too much to handle.
Like most of the users said in this board, Jigglypuff is 100% about bait and punishing the opponent, but has in return no way of approaching someone who does not want to approach you at all. So it's basically an incomplete form of bait & punish.
To me, the purest form of bait & punish would be Sonic, as Sonic is not only able to do what Puff is supposed to be about, but he does it even better with the fact that his speed makes him able to catch up to turtling characters, ridiculous spacing and attacking options, as well as easy follow-ups.
Jigglypuff has nothing of these. You either try your best to approach your opponent and risk to get blown off or basically wish that your opponent gets to you.