There are a few people competing with cockers in obedience
I hope you and Rosie continue to enjoy as it's really good fun!
Whatever people say you are not expecting too much with a cocker! They are more than capable of holding their own: I know of 2 that have won out of beginners and won novice classes in the last few years and several that have been holding their own in novice or the beginner classes.
It is harder for small dogs on uneven ground than bigger dogs, especially if they work nice and tight to your leg, but it is possible...a very stylish and motivated Staffie down our way recently won class C!
Yes, a sense of humour is a must!