No single color can be muddy. Mud happens in relationship to surrounding colors. If a vocalist sings a flat note or a guitar string is out of tune, the off-note by itself would not be offensive, but combine it with another tuned note or two and we cringe. The same is true for color. It requires a sour relationship to its neighbors to become muddy.
One way muddy color can happen is when the shadow values are out of context, meaning they don't relate chromatically to not-in-shadow values. We see it especially in light colored images such as Caucasian faces, white vases and snow. The shadow colors in this child's face are muddy, causing the child's face to appear dirty.
The mud is caused by a poor chromatic relationship. A chromatic relationship is a sequence of color values from light to dark (or dark to light) that follow how hue, value and intensity change as light on a shape moves into shadow or shadow to light. One doesn't simply add a dark color to create shadow. That color should relate chromatically or it will not feel like shadow.
In this next version, there is no mud. The chromatic relationship is right.