While it certainly cannot be argued that Catharine MacKinnon never would have found popularity without her notoriety as a lawyer, I believe it was primarily her work within the feminist movement that brought her so much attention. The feminist movement in general was (and remains) a very hot topic, and MacKinnon's work was instrumental within the community. Had she not been involved with such an important area, she almost certainly would not have found the fame that she did.
That Catharine MacKinnon was popular was a result of her feminism, not the fact that she was a lawyer. There are plenty of female attorneys. If all they do is practice law, they do not end up notable. It was MacKinnon's feminism and her positions on feminist issues that made her notable.
There are tons of lawyers and so many of them who believe that they need to talk and make popular their ideals, but the reason she was successful is because she is a feminist. Feminism, especially harder core feminism, has a built in audience that can attract people by pandering.
Catharine MacKinnon's books are about feminism, not about being a lawyer. She wrote about sexual harassment and founded a new branch of feminist theory in the mid-1980s. In 2006, MacKinnon asked, "Are Women Human?" in a mocking commentary regarding how far or how little women's rights have come in the world over the past 50 years.
I believe Catherine MacKinnon is more popular and more known for being a feminist over being a lawyer. While her education and profession made it easier for her to enter the arena of activism, it certainly is not the reason she gained notoriety. It is her ideals on feminist topics that made their way to the American public.