Nah, I've read plenty of interviews where both Justin and Dan have addressed the whole Spinal Tap/not-serious-band issue. The only things they don't take seriously are themselves and the rest of the depressed "metal" bands that do. Rock wasn't rock anymore, so they wanted to bring back fun, grandeur and guitar solos. They're a "you either hate us or you love us" band, and they're fine with that. 
