There is definately a pattern to it. I have won donuts many times at sideshow you always about 40 to 50 days apart. I never skip a day visiting sideshow you and pop balloons usually twice but sometimes 3 times a day. I started noticing the pattern late last year but I didn't start trying to keep count until last March.
21 March (wasn't counting)
2 May (293 balloon pops but I probably mis-counted)
13 June (301 balloon pops)
I'm at 156 ballons popped on this count and at my current balloon popping rate means I should win again around July 31 or August 1.
I don't know for sure but maybe skipping a day visiting sideshow you resets the 100 game/300 balloon counter which could explain why some people claim they have never won donuts there.