On one hand, it sounds good, as long as you can finish writing, polishing, and shopping around the story before the fad is past. {bite lips} That's the trick: getting it into shape and sold while the subject is still popular now, and hasn't fallen out of popularity. {bite lips}
That's why neither of my two major mentors liked writing for popularity. Both Vivian L. Thompson (author of many children's picture books and retellings of Hawaiian legends for all ages) and my father, Roger E. Baldwin (author of an introductory genetics text and Hawai'i's Poisonous Plants) taught me that trying to write to popularity is entirely too good a way to end up holding half-finished and half-polished manuscripts about things that used to be popular, but aren't any more. {half-smile}
They also taught me another approach to writing for publication. It's not as glamorous than being part of Today's Big Thing. However, it doesn't leave you stuck with Yesterday's Big Thing either. {Smile}
They both suggested following publishing's "conventional wisdom" until you've become established. The conventional wisdom is the set of rules that "everyone knows" in publishing, book selling and libraries. They're pratically never based on systematically gathered information, or even unscientific polls. They're usually explanations somebody thought of for some pattern of book-choosing they thought they saw. As such, they're very often wrong, sometimes ridiculously so. Even when they do have a point, they often miss something. {half-smile}
I don't know them all; there are a lot, and I'm terrible at memorizing. However, a few I've run into recently include "boys only read books about boys, while girls read books about both boys and girls;" "no one wants to read about handicapped people being handicapped;" "teens don't want to read about old folks in love;" and "Don't kill off a good guy in a kids' book."
When you're first trying to get published, it's best to follow most conventions whether they're right or wrong. Publishers and agents have heard them so often that breaking the conventional rules of publishing will usually make them look at a manuscript, and say "Well, I like it, but..." and rattle off which ever rule you broke. When you're new, you want to avoid that "but." It will make them set aside your manuscript "to think about it." It is possible they'll pick it up again, but they're more likely to reject it than a manscript they like that doesn't make them say "but." {half-smile}
You don't have to follow these rules forever. Once you're established, you can start breaking some of them. But first you have write a few successful stories. I think - and Dad agrees - that means writing at least three. The first story shows you can write a successful story, the second shows that you can do it again, and the third shows that the second wasn't just riding the coat-tails of the first. {lop-sided smile} If one of those
three wasn't as successful as you think the publisher would like, wait
another book or even two before you start breaking the rules. Still,
three, four, or five books isn't forever. Eventually, you can start
breaking rules, because the publisher knows you write books that sell.
Past success trumps these rules most of the time. {Smile}
So that's the method I was taught. I don't think it's actually not incompatible with trying to write something popular. You can do both at the same time or not, as you choose. Myself, I'll follow this. I rather not get stuck with a bunch of stuff that used to be popular. {Smile}
Anne Elizabeth Baldwin