I basically agree with your first two paragraphs as a whole. Sheik does not have a great DD (which, IMO, is among the strongest tools a character can have). She has decent run speed, and less than good aerial mobility. Where she shines is really the great speed and hitboxes on her normals, plus her conversions, edge guarding, and now grab game. I don't think that Ness is bad, per se, I just think that he's weaker than about 80 percent of the cast.
Very quick interjection before I continue. Yes, you're correct, I want Ness to have better and more reliable conversions off of grabs. The simple fact is Ness has one of the shortest grab ranges in the game, and no mobility to allow him to get in and get that grab. Pikachu and Sonic have smaller grab ranges for example, but they obviously don't have a problem with speed. What baffled me is that Sheik got her throw mix up back, which I think we all agree is a pretty strong tool, but a character like Ness, who has a much harder time controlling space, edge guarding, escaping combos, getting a grab, doing anything really, has had his throw changed twice now. I know that it's supposed to be an even line between 3.5 and 3.0 now, but there are so many characters with much stronger grab games that I really don't understand the point in changing it at all. Ness doesn't have a DI mix up, or command grab. Why should his only reward when he gets a grab be a fair that the opponent can SDI out of an receive 6 percent? I understand why Sheik got her mix up back, and that's not what is disappointing me.
Anyway, moving onto something else entirely:
It's my opinion that being able to kill from center stage (typically off of the top, with some high kb exceptions), is another very strong trait that a character can have in Smash, and PM specifically, due to the strength of recoveries. Take Fox for example: up throw>uair, up smash. Neither of those moves heavily rely on stage positioning. Fox can kill you from anywhere on the stage.
So, I made this chart. It compiles all of the data from various different moves and their knock back on Mario at 100% (using Strong Bad's knock back calculator). Let me know what you think (I'm about 90 percent sure that this is all accurate. But please let me know if you see a mistake):
It's important to note that these aren't necessarily a character's strongest moves, but rather, moves that either:
A.) Are typical KO moves for that character by being easy to set up into
B.) Do not typically rely upon stage positioning
To make myself even more clear, Mario's Fsmash is great at the ledge, but it's less than great going from left stage to right stage. His up smash doesn't rely on positioning nearly as much, so that is included, but Fsmash is not.
https://docs.google.com/spreadsheets/d/1O2FRviJavVXHQu1AVPGH4EEnV5KDWERAK-WoBuXrJuE/edit?usp=sharing
Something also worth mentioning is that some moves, like Snake's C4 detonation, or Fox's up air, may seem weaker than most, but they're usually hit out of a combo, where the opponent is already high up to begin with.