The stitch definition looks good. If you aren't already using a single stitch needle plate, that might help a bit along with using more starch/sizing before you start sewing. The stiffer the fabric, the more the thread will sit on top, rather than sinking into it. But the black thread on a shade of green might also be throwing off how clear the separation appears. You might try using the same thread with a pure hue, tone or tint for comparison so the black really stands out.