I don't really know how the Seat Belts fit in to the modern music scene. The Japanese have always been big jazz fans, and I suspect they must draw well there. It would be hard to believe all their fans there are kids.
My youngest is into the anime thing, and I think I've heard them as soundtrack on some stuff, such as the Cowboy Beebop CDs. I suppose that association could keep them from being considered very seriously in other quarters.
But I gotta say I like it. It kind of reminds me of the Oliver Nelson type jazz orchestra stuff. A whole lot going on. That alto player was killer.