I think the Cardassians are indictive of DS9 in general. DS9 was a real contrast to the cheery, black and white universe that TNG and TOS created. Cardassians, are imperfect, dynamic people. They're bad guys yes, but they also have moments when they show you that they're not just ruthless and domineering. More than any other race, Cardassians defy the typical Trek alien stereotypes, and are more akin to humans than any other race in my opinion
The Cardassian were created in TNG, and they were just as Grey there as in DS9 (and TNG was hardly all "Black and White", same for TOS). The reason they defy the Trek stereotype is because they were a recurring race and not isolated to 3 or so episodes which allowed better growth. The stereotypes apply only to the 1 episode wonders anyways.