"Middlebrow" is just an adjective like any other, and if you want to get into the class element, of course music fits into a discourse that would include adjectives like "middlebrow"...why wouldn't it?
Music isn't some kind of privileged other outside of class consciousness considerations.
Well I said it only mostly makes sense as a class thing, although even if it is from that angle it still is a useless discussion as far as I'm concerned when you're bringing in things like "confessing" you like it.
Middlebrow as a sort of value judgement though is extremely conceited. To say that Radiohead are definitely middlebrow because they are consumed by a large group of people who generally are all less enlightened and able to appreciate stuff that we all differently group as high-brow, but because of their relative sophistication are above liking stuff than the pedestrian shit we all differently group as low-brow.
I just don't see the point of this other than to self-conciously talk about certain musical groups/artists while still asserting cultural superiority/credibility.
EDIT: To be clear, I understand we all do this to a certain extent when making any sort of value judgement, but I think it's the large-scale grouping, the class connotations, and especially this idea of "confession" that really turns me off.