This is absolutely a very good step.
I would recommend separating in at least two certifications tough;
1) "nopCommerce certified backend developer" that focuses on the technical and backend side (knowledge of core and plugin development)
2) "nopCommerce certified theme developer" that focuses on the frontend part. (building templates the right way, javascript development etc)
In practice developers will specialize on either one of these areas. So it makes sense to have separate certifications. Besides it will help store owners selecting the right "resource" for their projects.