Lorna Simpson's work is not limiting, it actually challenges and confronts the narrow and limiting views that society has about race, identity, gender, and culture. She uses the African American woman in her work as a way to examine hoew gender and culture shape relationships and experiences in all of our lives.
Since Simpson explores the theme of identity and the problems of establishing, protecting, and defining identity, her focus on race is not limiting because the scope of her conceptualization is not limited since the theme of identity is as vast as the population.
Interestingly, I often think that the true limiting force on artists in every sense of the word is often critics and the way that the audience receives their works. As #2 explains, with any such valencies as colour, religion or sexuality, there is the temptation for critics to judge such work based on the artist themselves. I remember reading a really interesting article about Alan Hollinghurst, a British author who is gay, about how he finds it so restrictive being labelled as a "gay author." Perhaps in answer to your question we should examine the way that we pigeonhole artists rather than allowing them to define themselves.
This is a tough question. It's difficult because there will not be any definitive answers rendered. Rather, it is going to be one of those situations where the answer will actually open and prompt more questions. Usually, issues like race and art will do that.
Indeed, the subject of race is a challenging one for the artist of color. In many respects, I think it is similar to other valences like class, sexual identity, psychological identity, gender, or cultural difference. On one hand, it is it is challenging because the artist wants to define their own sense of reality. In a sense, they wish to go beyond the construction of what is present in society. They wish to transcend these limitations and seek to do so in their artistic endeavors. However, in seeking to transcend, the artist of color might open themselves up to manipulation from others in suggestions that race is not an issue worth exploring, or that there is no challenges with race in society, or that since the artist in question choose to move past it, every other artist of color should do the same. Indeed, this is a challenging position. No one should be in the position to "tell" an artist to what cause they need to pledge allegiance.
Perhaps, it is here where we can find some potential answers. Lorna Simpson, or any other artist of color, should be able to choose upon what they wish to focus their work. Black artists do not have to speak for what it means to be "black," in the same way that women artists do not have to speak to what it means to be a woman. Simpson neednt speak to the condition of being a woman of color. It is not anyone's place to tell the artist upon what they need to focus.
That being said, I think that Simpson is an artist that wishes to articulate her own condition in the world and use her art as a vehicle to bring this out. Consider her own words on this point:
I think maybe there are elements to it and moments to it that I use from my own personal experience, but that, in and of itself, is not so important as what the work is trying to say about either the way we interpret experience or the way we interpret things about identity.
It is here where I think that we can find some answers to the idea of the role the subject of race plays in her work. Simpson seens her work as a way to transform what is into what should be. Additionally, she sees her own experience as a part of this process. This means that she is not afraid to use it to speak of the issue of race and gender in America. This evident in her work Necklines, which speaks to lynching and the condition of race in American History. While the subject of race could be limiting for an artist if it is imposed upon them, Simpson seems to appropriate this topic herself and empower herself within the discourse by taking it as part of her own identity. In this, it becomes evident that Simpson seeks to empower herself and those who connect to her work because it speaks to "the way we interepret experience" and "identity."