While a personality disorder can be a mental disorder it is put into a separate axis to ensures "that consideration will be given to the possible presence of Personality Disorders...that might otherwise be overlooked when attention is directed to the usually more florid Axis I Disorders."

The separate Axis are really there for that reason. So that each factor can be considered separate. It is not required for Clinicians to use the Axis format either. They can just as easily write them all on one line. Obviously the APA encourages the former.