Not gonna comment about Sheik much. But I assume uair is really good against an air camping Puff. Also Sheik's jumps are fairly high. Also you can't seriously mean down+b is good on Sheik - that's just silly except like vs Pikachu and Fox.
Fox usmash kills Jiggs at like 80% or less. Also his uair is an accident waiting to happen at like 80% off the top if camping. He also fastfalls so much faster so just stay below Puff and Puff's air game is ******. Also if Puff is camping he has lasers and fine jumping skills. This MU is definitely not +2 Jiggs. At least +1 Fox. Also good Foxes won't get gimped by Puff that easily. I'm sure Fox and Sheik mains can say why the MU chart is where it is.
I just checked the MU chart on SWF and it says all the low tiers [Jigglypuff included] are -3 (Zelda and Ganondorf being -4) with MK. Unless you're telling me that they updated the chart, but not in that thread. The only character in the bottom 2 tiers who's -2 with him is Lucas. So no, Puff is -3.
G&W dies early from one attack? Okay land it then, cuz all of G&W's smashes should KO at <90%. Also he can just mash the b button if facing Puff and your DACUS will never land - all those sausage and the pan get in the way. And that's a really lame way to stop it, pretty sure G&W mains use stronger tactics. I'm sure they can explain why G&W wrecks Puff.
I am going to take an educated guess at this point. Sheik's Uair and jump height is not all that important. The biggest issue with jigglypuff is her side to side movement instead of the vertical height gained.
Take this situation for example as demonstrated above by the lovely image. Falco is in the black box while Jigglypuff is in the pink box area. Falco has the ability to reach jigglypuff, yes. His jumps and Bair would allow it to work. The issue here is that jigglypuff can drift very far left or very far right for an extended period of time. By the time Falco attempts to hit jigglypuff she could have completely driffted out of his range, but still have the drift to come right back and hit Falco near the apex of his jump with her Bair. Now, couple this with the fact that Jigglypuff has multiple jumps and she can hover in this zone for some time.
The point I am getting at is while Falco can enter Jigg's zone (the pink box area and slightly outside of it), Falco is actually weakest there compared to jigglypuff. Jiggs would prefer for Falco to jump up there. Falco's good zone would be him below a platform and jiggs slightly above the lower platforms. The reason is that jiggs is weakest directly underneath her.
Another strong situation for jigglypuff is a character simply sitting in shield (although stronger in melee imo). A character stuck in shield has greatly limited mobility. If Jiggs sticks to hovering around the upper right or upper left airspace of a person's shield with Bair and her jumps then few characters can directly attack her here.
-If you choose to sit in shield, then she can wear down your shield with spaced Bairs and you losing shield over time to eventually break.
-If you attempt to grab, you would lose to spacing since Jiggs disjoint on bair and hitting you from an awkward angle would cause the grab to miss.
-If you attempt to jump forward with an aerial you risk messing up by jiggs simply waiting around for you.
Again, what I am getting at is overall its not favorable to challenge jiggs in the air from shield unless you have the right tools. Most characters they do not have the right tools to do so.
Now, this is jigg's best strength. However, her downfall as well. She thrives on characters coming into her zone rather than her forcing characters into her zone. Once Falco gets a percent lead or stock lead there is no reason for Falco to risk himself going at jigglypuff when he can sit back and try to poke at her with lasers instead. And she has no strong tools to to force falco at him. The only thing she can attempt to do is use her mobility to try to corner falco in such ways that he gets stuck. Such as he might try shielding at the ledge.
In general, if Jigglypuff loses the position advantage all characters have to do is wait for her jumps to run out and have potshots at her landing. Or forever keep her to the outskirts of the stage. At the same time, I think Jiggs should be a bit higher on the tier list. I just think her play-style is incredibly tedious to pull off right while she only requires a few hits to get KO'd and many more to actually take a stock.