I don't think anyone who wanted, and was prepared to buy, a 4c has not purchased one just because of the lights.
Against that, 4 million is a lot of money on a car you'll only make 4,000 of every year. Once you've added margins, dealer mark up, VAT and other registration taxes, Sergio's horrible lamps could have saved you over £1500... There were two important targets for 4c: weight, and final cost. Going with custom lights would have broken the cost goal.
As the 4c is completely backordered for the foreseeable future, I think Sergio was right in thinking that the lights wouldn't harm sales, even if I agree with you that the lights could be nicer looking.