What does it mean when we call a person as a diplomat

A diplomat is a specific role, a representative of their country working in a foreign country such as an ambassador or envoy. However, we can use the term 'diplomatic' to describe someone who has skilfully solved a problem without causing a conflict, in which case the term would generally be positive. You could say "he is just being diplomatic" which could carry more negative connotations implying that someone was being less truthful or sly, but mostly I think it would be recognised as a positive word.

a diplomat is somebody who is a civil servant, an envoy, a representative, an attache, or an ambassador of your country to another country...