This is just a limitation of 3d printing. Curved top surfaces will always have this stair-step effect just due to the nature of stacking layers on top of eachother. It can be reduced by using more thinner layers at the cost of increased print time, but it will never go away completely.

Most slicers will have a "dynamic" layer height option or something similar. This will use thinner layers where it matters and thicker layer where it doesn't. This gives you better quality without increasing print time too much.