Just like you should not start building a house without a blueprint you should not start spending time on fancy graphics until you know your game works and is fun.
Making your game look good can be extremely time consuming. Imagine that you have spent hours adding details to a level for a platform game, only to realize that a jump cannot be made a it is too far or too high. In order to fix this, you will have to redo all the work that you have put in the level. This is why you should prototype first.
The objects you use to prototype will not be in your finished product so it can be as simple as using a basic cube. If you feel that the basic shapes that come with Unity are not enough you can quickly make more advanced shapes with Blender or the built-in tool ProBuilder: