Okay so to follow up on what I said earlier:
First off I realize that your list doesn't account for rankings within tiers. My first objection was that ZSS should be above everyone in your C tier, so since you didn't actually order it I can't really argue it. She should also be above GaW, seeing as how she has a much better matchup spread and thus better tourney viability. In your list, I would place her at the bottom of B tier.
However, Pierce I think your writeup shows a pretty bad lack of understanding of the character, that seems to be pretty common with many higher level players in general.
First off, you write dsmash is what makes her good, which will clue any ZS player into the fact that you don't know much about her metagame at all. That move is used to punish poor spacing, laggy moves, dodges, and certain recoveries. It is by no means spammed, and its utility can range from really really high to absolutely useless depending on the matchup.
Her overall speed is some of the best in the game, if not the best. She has excellent movespeed, as well as amazing speed on the ground and in the air.
You don't really mention her aerial game at all, which is among the best in the game. With sizable disjoints and good speed on all her aerials but dair, as well as easy damage wracking combos from nair and uair, this is the core of her gameplay. Her ground game works to support this aerial focus, as her two main tilts and best ground moves (utilt and dtilt) both put the opponent in a perfect position for aerial chases.
Her side b is an amazing move that, like dsmash, is very matchup dependent.
Basically what I want to stress is that with her combination of amazing speed, amazing range, good comboability/damage wracking, and good power for a lightweight, she has the tools to deal with every matchup and has more tourney potential than the rest of C tier as well as GaW and possibly a few other characters in your B tier. However, I'll agree with anyone who says she needs to be proven more, and for that reason I think that for now top of your C tier would be justified.