Opposite of The Beatles (B-Dulls). As well as this there is also the musical joke of there being no note called B Sharp (B#). This connotes that the group themselves are supposedly out of tune (Despite the reasonable vocals in the episode).
It could also be a simple pun on being "sharp", as in sharp dressers. The note B# actually does exist and occurs in the key of C# Major / A# minor. It is the same note as "C" but written as "B#" by convention.