Don't despise people just because they're poorly dressed.
Literal
Poor appearance [object-を] doing [just-because-からといって] people [object-を] despise [don't-な].
Combines からといって ('just because') with the blunt imperative するな. 貧しい身なり (みなり) means 'poor/humble appearance.' 身なりをしている means 'to be dressed/look' — using the ~をしている pattern for describing appearance.