Section 4.4 of Box2D manual says:
You must create polygons with a counter clockwise winding (CCW). We must be careful because the notion of CCW is with respect to a right-handed coordinate system with the z-axis pointing out of the plane. This might turn out to be clockwise on your screen, depending on your coordinate system conventions.
If you are taking a shape and mirroring the coordinates you are probably changing the winding and that is the reason it fails. But as you didn't provide an error log (the box2d c/c++ assertion) I can't be sure.
I believe you can perform the mirroring you want if you invert the array of vertices too, before creating the box2d shape. What I mean is, if you have [v1, v2, v3] then by inverting the array you will have [v3, v2, v1].
Hope it helps.