Well I'm not American, but I doubt Maria is a nobody, that said a lot of people do overstate her fame. But she has had a few stints on Letterman, Ellen and I'm sure she got a lot of coverage for Wimbledon and I'm not sure what her coverage for the career slam was like, but it was big over here. Ironically the Canon commercials were the only real U.S. thing she did, a lot of her endorsements were Asian based surprisingly. A lot of my friend who I wouldn't really talk to about tennis know her because she's 'fit' apparently, while just have many has said "that one who screams all the time" or "that one who one Wimbledon". It applies to everyone though, to my astonishment I've yet to meet someone who outright knows Henin, and even with the WS I get responses of "them black girls" or even if they know them they have to ask who's the 'big' and who's the 'skinny' one. It's hard for us as tennis fans to really grasp the players fame outside the sport, as we don't know everyone. But all fanbases like to overstate their fave's fame.
I watch a lot of ESPN, and I don't recall them ever mentioning that she won the career slam. They did show on the ticker that she won the final but never mentioned the career slam. Before the final, SportsCenter was periodically showing highlights from her SF against Kvitova, and mentioning that she would play Errani in the Final, without showing any highlights from the Errani-Stosur SF, so apparently they did think her SF was more worthy of highlights than the other SF. After the Final, they showed highlights of that periodically, but again, I don't recall them ever mentioning the career slam.
They showed highlights of the men's SFs and F much more frequently, and whenever there were rain delays, both ESPN and NBC chose to show reruns of the men's matches rather than show reruns of women's matches. One day, they showed reruns of both of the men's SFs twice. I don't have tennis channel, so I can't speak for what they did.