外人 has a derogatory nuance, because it's curt (therefore not polite) way to say something which has more polite forms, such as 外国の方 (gaikoku no kata) which is a respectful way to refer to a non-Japanese person or people. Depending on context, of course. In some contexts and situations, there is no respectful way to point out someone's nationality, such as when it is completely unnecessary and irrelevant.