Her official biography says "Catharine Alice MacKinnon is an American feminist." It then lists some other impressive credentials. Considering that the word "feminist" appears first, and considering her tireless work to combat pornography, sexual harassment, and wage disparity I think we can safely say yes. Feminism is most definitely her work's main focus.
That is her entire point, if you look into her entire body of work and all of the causes that she speaks about. She has taken a very hard line stance against prostitution and against all forms of pornography, especially because her arguments about these issues are purely from a female perspective.
Yes, Catherine MacKinnon is a feminist who writes a lot about Feminism. Her books deal with a lot of issues that involve women or that women face including sexual harassment and pornography. She takes an anti pornography stand which is a stand that some feminists take, and compares it to human trafficking, but it's far from the stand a majority of feminists take.
Yes, feminism is the main focus that Catharine pushes out in her works. The reason I believe this is because she has spent her entire life working as a feminist scholar, and she is trying to distribute her findings out through her books, so that we will follow her advice.
Looking at Catharine MacKinnon's body of work it is obvious that her works mainly focus on feminism. In fact, the reason she was labeled as a feminist was the content of her works and the topics she covers. I believe it is important to realize that while she may have fought for what she believe in, it does not indicate that all women agree with her in every aspect.