Art is subversive. That's why oppressive regimes censor free speech and artists of all kinds. I'm taking this as axiomatic, without going ahead and citing references.
But there is a line between art and activism. The real question is where that line exists. Has Michael Moore crossed that line - are his documentaries too closely associated with his clear activist goals? Hollywood actors often have clear political convictions they like to speak on. Bono, from the band U2, uses his fame to promote his own social justice agenda. I'm not criticizing any of these people. But as artists, I think we need to consider the line.
As a writer, I have something I want to say. I happen to want to say it using fiction, in a variety of ways. I find myself drifting towards extreme topics - dystopia, tragedy, personal dysfunction. But these are the situations where my message finds itself. But I flatter myself that I am keeping my personal activism out of my writing - the story is dominant and the message is defined by the story. My characters do things I wouldn't agree with. Not to mention, there are many areas where I am passionate, and I just can't cover them all in one story.
To quote Anne Lamott from Bird by Bird again, we should write about something. We don't want to write flabby stories where nothing happens, where there is no point. But we also need to be aware of the line between art and activism.
I just finished reading Little Women, and let me tell you, Louisa May Alcott had a point and she didn't hesitate to make it, over and over! Little Women is chock full of morals and life lessons, to the point of being didactic. Yet that doesn't take away from its literary value, because it was written within a certain historical context. We can't afford to be that didactic today. Yet I don't think we should compromise our message either.