Old spark plugs that are worn or have been in the engine 100,000 miles or longer can cause a decrease in fuel mileage as well as cause poor engine performance and loss of engine power.
When the spark plugs are worn the fuel won't be ignited and burn properly which increases fuel consumption.
So if your engine is suffering from poor performance or loss of engine power or your fuel consumption as risen then yes replacing those bad spark plugs with fresh new spark plugs can improve your vehicles fuel mileage.