Define popular. ls it as popular as other football code sports like gridiron football or soccer? Not even close. But Rugby USA has 100,000 registered players and 2,500 registered clubs. Every community big enough to warrant its own postal code will have a club. So it is known and played everywhere.
As a foreign student in Florida. I've noticed that there are many rugby teams in town as well as the university rugby team. I would say not as popular as American football, but if you involve yourself in rugby activities there you'd be ok.