I looked up Q scores because I didn't know about them; but I couldn't find much.

I think there is little knowledge of other Supremes outside the fans; it is one of the facts that galls Mary's fan base. But I think polling shows what you say and I think it has become significantly more pronounced in recent years.

That is not to say it is deserved but lots of factors are at play. I think Cindy Birdsong was identified as a Supreme more than Mary Wilson was in some polling done around RTL time and that likely had more to do with having a generic name like Mary Wilson versus a Supreme songbird name like Cindy Birdsong.