When a baby is first born, his eyesight is not always the best. Think about it. He's been confined to a dark place for nine months and all of a sudden he's forced out into the bright new world. I'm sure you'd be a little squinty too.
After reading our baby books, we found out that newborns see a little differently, so we based our choice of crib mobiles on this:
Shapes - Simple shapes are easier to distinguish than more complex images.
Color - Black and white is easier to see than colors in the first few weeks of life - as are gradients of black and white like grays. After that it's more about bright primary colors.
Distance - At birth, newborns can only see about 15 inches in front of them, a perfect distance for seeing the faces of those holding him.
We ended up choosing a black and white mobile that had a fun circus theme - zebras, pandas, birds and more. He seemed to like the checkerboard print and the stripes. Everything was in very high contrast, so he could see it easily. As he got older, we also started showing him pages of board books that had similar colors and along with shapes and patterns.