Do you have a link to the survey? I'd be interested to learn about their polling data. Personally I think the data is skewed. Carol is popular, but there's no way she would poll above Iron Man or Thor, both of whom have had their own movie franchises.
Mind you, in May 2015 we had a female Thor and an inverted Iron Man, so who knows? The timing may also play into why Black Panther scored so high. He had some great moments in Hickman's Avengers. His feud with Namor (who I hate) was a high-point.
I found this on a site called comicsalliance.com.
You can find it if you search for Nightmask there as I found the survey results at the bottom of an article about the new Starbrand and Nightmask series. I had a lot of trouble navigating this site as my iPad slowed to a stop. Might work for you, I guess.
The slowness of it led to me not lingering long enough to see who got polled for this. Good luck.