The MLS exploded in popularity lately and still stays below the NHL, the smallest of the big four leagues, and they jusssst passed NASCAR. Football is ubiquitous here, baseball and basketball are popular, hockey can be pretty niche, and NASCAR relies on certain regions and is basically dead in others. Compared to them, I think its still fair to call it not popular. When people think pro sports here they think Football, Baseball, Basketball. Thats the bar.