You can really say that about any character though (except falcon, he's just terrible). some ness players can often beat out the higher tiered characters without problems. I think tiers aren't really the best way to judge a character's skill, because it's how the player uses that character that shows whether that character is good or not. other than that, ZSS will maybe be low B tier, Ness SHOULD BE either C, D, or E tier at least, ICs are going to eventually be high A, and captain falcon is CHUCK NORRIS TIER!! (SSSSSSSSS).
I understand what you mean. But all those characters have innate weaknesses that can't really be avoided: Ness = Easily gimpable recovery, for example. That, in reality, can't be controlled. No matter how good you are, all you can really do about this is learn to DI to avoid having to use it, but then that's something all characters can do, so it only factors in slightly more than usual. As for higher tiers, they are lacking in these unforgivable moves, or can just cover them well.
Now, what I'm saying about ZSS is that a very large amount of her weaknesses can be covered by skill, but for the most part, only a very large amount of skill. Recovery, for example: It's easily gimpable if you don't know what your doing. After you get better, you know how to incorporate plasma whip and flip into the mix, making you recover much more easily. This isn't a great example, since it doesn't take a substantial amount of skill to reach this level, but the idea behind ZSS, as shown by her gargantuan learning curve, is that once you know how to use her really well, you cover these weaknesses up to the point that she gets much,
much better.
More examples; MK has plenty of weaknesses, but they are for the most part (if not the all part =P)very easily covered by his spammy attacks, etc., so if he is played right, he easily ***** everyone. ZSS also has weaknesses, and a lot of them, but a large majority can be covered up, so she can played, in my opinion, very very well, almost high-tier type play. But, it takes a looot of skill to reach a level this happens, because it is complicated and difficult.
Keep in mind that to really start hitting that up curve takes a huge amount of practice. I don't think I'm anywhere near it, and I'm pretty sure that only a few people are really around the up curve of that line.