Is a genetic mutation considered a disorder only if it causes harm? or are beneficial or harmless mutations considered disorders as well?

Depends: Interesting question, but fundamentally a semantic one. Harmless mutations are usually called variants rather than disorders. Also, external factors might make a mutation useful in one circumstance but not another. For example, the red cell mutation called G6PD deficiency can cause (mild) anemia, but in places where malaria is prevalent, it is somewhat protective for that disease.