Obviously you need the right design at the right time, but it doesn't cost much to get a shirt ready for production, so if inspiration strikes it can certainly be financially rewarding to ride that pop culture wave.
If you have the channels to get it out there I would say yes but you definitely have to strike while the iron's hot. A friend and I waited too long and ended up not doing our "BRITNEY IS THE NEW WHITNEY" shirts because she dropped out of the news.
I've used cafepress to put a t-shirt into production about 15 minutes after I read about a news story and had profits coming in within the hour (using Google Adwords to drive traffic to the design).
I know of at least one store that made $100,000+ with a timely political design.
It can be time consuming to keep up with it (I stopped), but it can be profitable for sure.
Trend T shirts... its an option? 
