The Sportsman sequence here in the States is specifically designed so an airplane without inverted systems can fly it and be competitive. Yes, there are some short periods of descending inverted flight (half-Cuban for example), but the sequences do not require sustained inverted flight.

A Citabria is very capable of doing just fine in Primary or Sportsman...but you will be working harder than the average Pitts/Extra pilot. I also completely disagree with Diana on key one point, a well flown Citabria can beat a Pitts/Extra almost every time. A Citabria will get better scores because the judges can see you better than a little football shaped S-1 at the same altitude (~34 ft wingspan vs ~17 ft wingspan). But then the judges can also see each and every mistake a big Citabria makes...so don't make mistakes.

The primary drawbacks to using a Citabria is the limited roll rate and lack of inverted systems. Not having inverted systems can be managed by adjusting slightly how you fly figures, and the roll rate is mostly off set by the slower speeds a Citabria will have in the box. An Extra cooking along at 180+ kts needs the higher roll rate to get the same figure done in the same space as a Citabria only doing 120 kts. And the slower speed of a Citabria can be a big advantage...for example this year's Sportsman Known has 2 downwind 45's on the same pass. No way most pilots will keep that in the box, but a slow flying Citabria or Cub has a good shot at doing it. The slower speeds also allow the pilot to put the whole sequence right in front of the judges to maximize the opportunity for good scores.

If you want to see a well flown sequence in a non-inverted airplane, watch Giles or Greg fly their Cubs in a contest. These airplanes also have lots of adverse yaw (so lots of footwork required) and need more nose-up attitude when inverted than most pilots are used to, but they still score and place well in contests.

Get a set of spades for the ailerons if you don't have them, makes a huge difference in flying good quality rolls and reducing pilot fatigue.

As for doing well in contests, it doesn't matter what type of airplane you are flying. If you don't have good critiquing from a qualified observer on the ground, you will not do well. Practice sessions with feedback from the ground are absolutely critical. I fly perfectly round loops every time... the judges just don't seem to agree.