The belly of a frog or minnow are pretty stark looking, flat white, with not too many features.

Also remember that in most cases a bass isn't simply going to be viewing the bait from directly underneath, but at an angle.  At an angle, he will have a view of the side of the bait and will see the eyes, the scales, the markings, etc.  I do agree that the bottoms of baits should have some more detail, but not much more.

I think I posted this before, but, the way I buy topwater lures is....

Natural colored bottoms. Yellow, white, green, black. Natural colors for a fish to see, if the fish sees it.

I buy different colored/patterned tops, so I can see them while working them. The brighter the sunlight, the darker the top. If the water reflection/background is bland, I use patterned top. If the background/water reflection is broken up, I use a solid color. I like to visually work topwater lures.