Q: Why is Ness so low on the tier list?
A: Ness dropped because his DJC isn't broken due to bad aerial hitboxes, his approach is piddly due to the same reason, he has mediocre grab range and the attacks with passable hitboxes are the ones that don't see much use for one reason or another. Overshadowing all of this is the worst aspect: his recovery, which is absurdly predictable, slow and edgeguardable to a fault by anyone half-decent.
Q: Why is Jigglypuff so low on the tier list?
A: When it comes down to it, she has poor matchups against some of the worst characters in the game, which is why she has been dropped. The tier list was mathcrafted, and if you do poorly against even worse characters, you won't be high up. Jiggs certainly has good combos and the ability to 0-death a good majority of the cast, but she lacks a couple things. The first is range - her only notable attacks with range are N-Air and perhaps D-Air. Everything else, including her grab, U-Tilt and smashes, has either piddly or mediocre range, and as such her approach is limited.