I'm not exactly sure if such a thing exists in the wikia stats, and I think you can bring up poll history just by going through the edit history on the front page, but I may as well make this resource here so that it's somewhat easily accessible.
March 30 2015 - May 31 2015
Who is your favorite support character in K-ON?
- Ui Hirasawa: 146 (63.48%)
- Sawako Yamanaka: 39 (16.96%)
- Nodoka Manabe: 24 (10.43%)
- Jun Suzuki: 11 (4.78%)
- Other: 6 (2.61%)
- Sumire Saitō: 3 (1.3%)
- Nao Okuda: 1 :( (0.43%)
- Total: 230
Wow, Ui by a wide margin took top spot. Not particularly surprising since she does share quite a lot of the screen time. I think Sawachan has had more screen time than Ui overall, but I guess Ui wins just by being the adorable, perfect little sister she is.
I personally voted for Nodoka, since she was just the best counterbalance to how hectic the Light Music Club normally is, but she still has just enough leeway in her character so that she isn't just 'strict and nothing else'. I feel Nodoka works on a few more dimensions of character than Ui does in this respect. I don't feel Nodoka is as big a trope as Ui is, and at the end of the day, Nodoka is just one of the most sensible characters in K-ON (and yes, that is a compliment).
Out of the top four, Jun makes most sense to take fourth place. The anime plays a bit too hard on her 'Mio fangirl' angle, and will bring it out even when it's utterly contrived. Jun becomes a lot more enjoyable in the last two volumes of the manga, where they flesh out her character a lot more, since there's more room for her character to interact with others.
I remember that some of us were a bit iffy about who really counted as a side character. I drew the line at Sumire, since she seemed to have a lot of fanart, so I thought people would be interested in her. I added Nao because I thought it would be unfair to include Sumire, and not her. With only 3 and 1 votes to Sumire and Nao respectively, looks like they had no chance of going up against the anime support cast.