It would be great if there was a client config option so that the eyes only showed up above mobs while you were stealthing instead of either having them on all the time or off all the time.

why?

In games like Skyrim, when you're standing upright then you're generally not trying to sneak. When you enter sneak mode (which in Minecraft is crouching) that would mean you're trying to be sneaky and the eyes being above their heads would be useful at that point.

If nothing else it gives players who don't like the eyes always being there a way to enable them situationally, and I know Minecraft has some sneak mechanics involved while crouching so generally I'd imagine characters who are trying to be sneaky are also crouching.