No, niche means it doesn't have wide appeal to a wide audience and does not include mainstream viewers. I am not saying "nerds" is a negative word either. I am also not saying that a hit science fiction show can't draw an audience. I am saying this show doesn't have lasting wide audience appeal outside of science fiction fans IMO. That applies to MOST science fiction shows. You are not going to get mom and the family sitting down to watch this which to me is mainstream every week like some Star Trek shows, Star Wars or even Last of us. It's doing well out of the gate but many shows do.
It won't last IMO. I could be totally wrong and this show does well. We shall see.