Funny you sat that.
When I first watched k-on I thought she was annoying and uninteresting while I found Mio cute and all.
It wasn't until my second rewatch that Ritsu grabbed my attention and slowly started enjoying her appearances. After completing the series the second time I fell in love with her.
Another thing I attribute to me loving Ritsu during a second rewatch was the episode A Winter Day, which I skipped when I first watched K-ON because I thought there were only 12 episodes in season one.
I think that episode alone gave me all the reason to love Ritsu from that point onward.