Having written more than my share of material about stuff I didn't care about, I don't think it's important as far as earning money goes. The world is full of jobs that aren't fun and that people do because they need to earn a living.
Where I think liking what you write about becomes important is when a person does nothing but write stuff he doesn't care about, or does that for too long. Somewhere in the mix, I think there has to be some writing (at least for a person who thinks of himself as a "writer", not a "churner-outer") that the writers cares about. If there isn't, after awhile it just gets mood-drainingly tedious to the point where a person can't stand to write another uninteresting/unchallenging word.
My best Hubs aren't my best writing by any means. Then again, my most well written Hubs do well (usually in ways other than earning).
I guess I just think if someone is using the writing skills he learned in high school to churn out articles it isn't important that he likes the subject. People who "have writing in their hearts", though, need to like the subject at least some of the time.