Theoretically yes if you use them. In practice, no. The OEM fog lights don't change the car's aero shape but they do draw power when used. Anything that draws power theoretically reduces fuel efficiency.
I would imagine that the difference in MPGs would be minimal. There may be a bit more drag, and they do use a little more power when they are on, as mentioned above. I track my MPGs on a spreadsheet. There is about a 1MPG difference before and after, but it's hard to know if that is due to the fogs. I had them installed when my Cruze was only a couple months old, and my drives weren't the same before and after.
I highly recommend the OEM fogs. The fog lights increase visibility. I use them all the time at night, even when there is no fog. They make the front of the car look much better both during the day and at night. At night the fog lights look cool. During the day the front bumper looks complete. Without them you just have ugly black filler plates that look like something is missing.
70 watts for the pair, when on, about 0.094 HP extra load on the engine, a lot less than the radio or #1 speed on the blower motor.
Many have installed their own, including me, was fortunate to buy an OEM kit from a dealer on ebay at auction for $130.00 when one else bid on them with free shipping. Should be standard on the Cruze, the side lighting is terrible on this car, would only add about ten bucks at most to the vehicle. Sure have a bunch of worthless parts left over.
In terms of fuel economy, worse thing is driving into a head wind, only drive when you have a tailwind. Yeah, sure.