DMB, definetly frat rock. Them and The Offspring and Franz Ferdinand. Still pretty good stuff though. Even though I'd never be "one of them" (fratboys/sororigirls) it's still fun to get that kind of a feel going sometimes.
Though... According to Wikipedia frat rock was a genre of rock strictly from the 1960s. Can't say I agree with that at all. The way they're using it, it's just a synonym for garage rock.
Immature? Nah, I think it fits perfectly. It's not a diss, it's just a style. Frat guys like them as often as metalheads like Slayer. It's not universal but it makes sense.