By saying they don't know nor care who Nightwing is, you are implying there is no interest among the mainstream and that it's at the same level as yours. You're deflating Dick because you don't like him, coming from the opposite end of the extreme fans.
The truth as always is probably somewhere in the middle: all fans exaggerate how successful their favorite would be, but really of DC's characters the mainstream only pays attention to Batman, the Joker, Superman and only recently Wonder Woman to a lesser extent. But Nightwing has a starring role in Titans and Young Justice which increased his profile, and those who keep track of such things better than I do say merchandise with his logo moves a lot of units. So there probably is a little more awareness and interest among the mainstream than a lot of other DC characters or Bat-family members.