There are some not-so-nice areas near the Gare du Nord. Most people in the station aren't from the neighborhood, they're there because it's a major rail hub (metro, commuter, regional and inter-city) that handles 180 million inter-city rail passengers alone per year. Compare that to the 4.3 million that New York's Penn Station sees, and you get a sense of how big this is.
I haven't been there in a long time, I must confess.