Dawkins would be reluctant to use the word design in the first place, and he wouuld measure fitness not by your ability to see it, but by using a quantitatively differentiating reproductive and survival values.
Your straw man arguments aren't even worth discussing. YOu start with your premise that flatfish are bad design and then ask others to justify your opinion.
BTW if you cared to do any actual research you would find fossil evidence of transitional flatfish.