Minecraft's is a universe made up of square blocks. But square blocks can be assembled in a variety of shapes, including spheres.

The Sun and Moon, even if they were made entirely out of the meter-squared blocks that make up the Minecraft world, would still appear round to the eye of someone far away from them. I don't think that they ought to be entirely smooth, but a pixelated circle would fit nicely and make sense in the context of the game.