I don't care about people who want to force sub genres for the purpose of sounding like an elitest jerk, but when I ask someone what genre fun is, I would like an intelligent and well-thought answer.
Rock is anywhere from Alexisonfire to ACDC to The Black Keys in my book.
And I just don't think fun is rock.
Let me ask you this:
Your ultimate question: There is this band, Band X. I want to know how their music sounds.
If you asked someone what genre Band X is in, and they said, "rock," does that really help you? Does that tell you anything about how they sound? Generally, maybe. Specifically, not really.
If you asked someone how Band X sounded, and they say "Oh they sound a bit like Band Y, and a little like Band Z. They have an acoustic foundation with piano and even some electric guitar, with catchy melodies," then you have a lot more detail than just a genre.
My point: Genres are becoming pointless and really don't do any band justice because they all sound different anyway. So when inquiring about Fun, you shouldn't ask for a genre.