Whether copper is not good for accuracy -- its presence probably has no bearing on it -- is probably less relevant than its lack of usefulness on match grade ammo. Copper plating was introduced in about 1927. While it has lubrication, it was promoted as "greaseless" and "non-lubricated" ammo which would not leave grease on fingers or pick up lint or dirt from pockets, yet still prevent leading in the bore. For these reasons it was attractive to hunters and plinkers. It's been used on ammo made for these purposes, which doesn't require match ammo accuracy, and which is less expensive than match ammo.
According to George Frost in his book on ammunition making, paraffin is not itself a good bullet lubricant, having a tendency not to stick to the bullet surface. The lubricant used on copper plated ammo is applied by dipping the bullet in the lubricant or by using a solution with a solvent that dries away. While the match ammo makers don't use the same lubricants, they are typically more lubricious than hunting and plinking ammo lube because that quality contributes to greater accuracy.
