Is cuckolding popular

Data on sexual practices is notoriously unreliable. But according to most U.S. research a solid majority of Americans have had extramarital or extra-relationship sex. Other research suggests that it is a common fantasy for both sexes. So in that sense, you would have to say yes.

More complicated is the issue of cuckolding as a fetish. It is a not uncommon fantasy, according to surveys, and according to recent big data analysis of web searches, much more prevalent then previously assumed (or admitted to by survey takers). But the number of people actualizing it as fetish play experience is probably quite small as a fraction of the population.