Ritsu and Mio are practically glued together, it's rare to have a scene with just one or the other, which might be one reason why there's a lot of fan art of them taking their relationship a bit
further.
I think K-ON!'s more like an ensemble, much like Azumanga Daioh, the girls get roughly equal screen time and focus as opposed to a story with a clear protagonist. Mugi gets comparatively less character focus, but she's still present during the girls' activities even though she doesn't talk as much. In other words, none of the characters really dominate.