While there is a small problem with the contrast of this piece, what really hurts it is the lack of depth; everything is very iconic and flat.
When you're working with smaller pieces of pixel art, well any size of art for that matter, it's very important that you worry about details last. You want to focus on the form, depth, and, lighting first.
Here's a sloppy edit:
The first thing that jumps out at me with this piece is that there is very little atmosphere; the colors looks pretty straight and flat, like a road.
It can help to try to imagine
where your art is, like in my edit it's in a dry place, a little bit like a desert because I've lowered the saturation and gave everything yellowish tint. This might not be what you wanted in this piece, but it's just an edit to show how you could give it a little more character.
Second, you're not really looking at the body under the dress, this is most evident with the skirt. The folds look as though they were just kind of thrown in there to break it up a little and not because of the tension the cloth would be put under in her current stance. Her face is too iconic for my tastes with such a small surface to work with; as you can see in my edit I've simplified it quite a bit, I removed the mouth because I've had this problem before and found it can look much better to just hint at a mouth than actually try to draw it with so little space. The arms are also lacking any coherent form; I can't say I'm very good at shading arms myself, so you and I should study how to shade arms more effectively.
Third, you haven't really shaded anything with as much care as is needed at this size; the shading is probably the best tool to convey detail without having to actually draw the object, which is a skill that can take some time to develop. What you have at the moment is highlights and mid-tones, but no shadows anywhere in sight; this is probably why a lot of people have said this piece is lacking contrast. Remember that shadows are a little different than the other two of the triad because instead of nice curvy ramping there is a flat void, if you will, where an object is blocking the light-source that would have illuminated it like the rest of the object; in short, the shadows are where the light just can't hit, so these should be left flat, but very dark. Look at the hair right next to the head, and just under her breasts to see how drastic shadows can get.
In closing, I'd like to point out that I have not touched the clouds and sky, and the rocks I just played around with a tiny bit; you can ignore these.
I hope I've helped you out in some way, happy pixeling!
