About the neck, I honestly think no one will notice. A 3-pixel wide neck centered on the chin is ok. But since it bothers you (and we pixel artists are detail oriented, so why not) I tried to come up with a solution.
Possible problem: A 3-pixel wide neck is too narrow, 5-pixel wide neck is too wide, and a 4-pixel wide neck is not symmetrical.
Solution 1: add a half pixel to either side of the neck. Wrap the neck and shoulders with the red vines, and add a color that is half skin, half vine. Use this color for the extra neck pixels and the result would look like a 4 pixel wide neck.
Solution 2: Use a 4-pixel wide neck, but turn the torso so that the neck is no longer symmetrical. This is what I used in the edit below. It's a bit of a trick, but it works ok. It also lets you be a bit vague about the length of the neck since the torso is not on a flat plane towards the viewer.
Solution 3: Ignore the thin neck. If you really want to distract from it, let hair grow long behind the head, filling in the gap between jaw-line and shoulder. No one will notice the neck dimensions at all.
Other comments:
You mention that the figure is tall and willowy, but she's got giant breasts. That is unusual for tall and thin.
I think the problem with the tutu is that there is no body underneath. No hips or belly to help shape the thing. It's also caught between being hanging down like a skirt and sticking out like, well, a tutu.
I find it difficult to use less than 4 colors for round forms like skin/body parts. I like light, medium, medium-dark, and dark/edge.
A couple of (partial) edits.

The first is just the figure you have with the torso turned to the side. This keeps the shoulders and giant breasts. The groin is raised and the torso shortened, with the legs sketched in at the hips about where they would go.
The second edit is based on the first, but it is thinner. The face and hair were reworked to keep with the thinner look and highlights added to the torso .
Hope this helps,
Tourist