As long as they like it they can eat it. Mine don't like cabbage, they love cooked carrots, and they love cucumber (the middle with the seeds). I don't give them the peeling. I think it is too tough.
Mine LOVE cucumbers! I just slice them in half length wise and lay them on the grass (so that they don't get covered in dirt). All the hens leave is the skin with a bunch of holes pecked in it!
I've never fed them cabbage, but I've heard that hanging a cabbage from the ceiling is a great distraction for when you add new birds. So I guess they must love it!