I'm not a veterinarian, but I don't think that breast feeding on a lactating cat is very healthy for a duckling. Lactation and breast feeding in nature is limited to mammals. Birds are not anatomically nor physiologically designed to do it.
I don’t think it’s breast-feeding at all - just resting its head on Mama’s belly.
I agree with you. Not only are birds not physically adapted to nurse, but I’m sure that the resultant milk will not be in line with their particular nutritional needs.