Having a controversial life or political viewpoint does not necessarily make someone a better writer, in terms of multicultural writing or any other genre. As the previous answer noted, first you have to define what good literature is, and in this case what good multicultural literature is. Good writing should be creative and effective in terms of form and content and could/should express ideas of a universal or current appeal. Good multicultural writing should showcase these same qualities with the added quality of expressing and investigating the customs, innovations, and qualities of more than one culture.
So, some controversial perspectives might give a writer some help in coming up with "good" multicultural writing. Consider the possibility of a writer who lives in Jerusalem and is a practicing Muslim and Jew. This seems like an odd combination. But for the sake of the argument, such a position, practicing two religions that (in that area of the world) have been in continuing struggle, is a controversial position. But such a position would give a writer a unique perspective on two different cultures.
On the other hand, if a writer happens to have a controversial political perspective that is limiting in terms of multiculturalism (i.e., a racist or nationalist) that writer is less likely to write a text that gives fair attention and appreciation to cultures beyond his/her limited cultural perspective.
So, having a controversial viewpoint is only helpful in writing good multicultural literature if that viewpoint embraces an open-minded perspective, one which is interested in different cultures and ideally, one which doesn't necessarily privilege one culture over another.
Also, a writer conscious of multiculturalism should pay attention to pluralism (where cultural differences are recognized) and assimilation (where cultures are forced or encouraged, by the powers that be, to coalesce together to form one culture). American culture is an interesting case study here because it is made up of many different cultures but there is a push by some political fundamentalists to assimilate, thus losing the diversity of a multicultural nation. (Many cultures can be unified under a common country, but assimilation indicates a homogenization of those cultures.)
A good multicultural writer would therefore investigate the plural aspects of a society's different cultures and would note the danger (loss of diversity) prompted by assimilation. And once again, the question depends upon the controversial viewpoint. The controversial writer who favors assimilation would be less likely to write "good" multicultural literature while the pluralist would be more likely to write "good" multicultural literature. And some readers might find the pluralist more controversial than the writer interested in assimilation; and vice versa. So the very idea of being controversial is also relative to the reader's political perspective.
This is an interesting idea, but it does not seem to me that having either a "reputation" or a "cause" would make one a better writer of literature. Good literature is defined as "writings having excellence of form or expression and expressing ideas of permanent or universal interest." There is nothing inherently better about a work of fiction, multicultural or not, written by someone who has a "controversial life or political view" than a work written by someone less controversial or political. Good literature is written by authors who pay attention to the craft of writing as much as to the content.
What is probably true, though, is that an author such as you mention would be more likely to have a platform and an audience for his or her work. In fact, for those who share the author's beliefs, the work of fiction would probably have greater credibility because of those presumably publicly held and controversial beliefs.
The most effective multicultural literature seems (at least to me) to come from those who have a passion for their culture and write it; passion, rather than controversy and politics, resonates with readers of literature. Note this example from Song of Solomon by Toni Morrison, a woman who certainly has views about race and culture but whose views would probably not be considered controversial (though the content of her works often is).
“See? See what you can do? Never mind you can’t tell one letter from another, never mind you born a slave, never mind you lose your name, never mind your daddy dead, never mind nothing. Here, this here, is what a man can do if he puts his mind to it and his back in it. Stop sniveling,’ [the land] said. ‘Stop picking around the edges of the world. Take advantage, and if you can’t take advantage, take disadvantage. We live here. On this planet, in this nation, in this county right here. Nowhere else! We got a home in this rock, don’t you see! Nobody starving in my home; nobody crying in my home, and if I got a home you got one too! Grab it. Grab this land! Take it, hold it, my brothers, make it, my brothers, shake it, squeeze it, turn it, twist it, beat it, kick it, kiss it, whip it, stomp it, dig it, plow it, seed it, reap it, rent it, buy it, sell it, own it, build it, multiply it, and pass it on – can you hear me? Pass it on!”
While this is obviously a culture-specific reference, it is a passionate call to action which can appeal to a wider audience than just African-Americans in America.
Many of the finest examples of multicultural literature were written by authors who did/do not espouse controversial or overtly political views. Perhaps it is because these things are usually louder, but not more effective, than a quieter, well written story.