No, in fact most of them are not, especially at the lower weight classes. In the heavy weight divisions you will see very large people who appear to have excess body fat, and to be sure some do, but most are heavily muscled and sometimes that can exaggerate their subcutaneous fat stores around the gut.
In actuality, many of these heavy weight power lifters’ body fat is within healthy a healthy range. That said, like football linemen, sometimes a little extra weight can give certain individuals an increase in strength and leverage. Power lifters that have to make weight classes below the heavy weight divisions are almost all stronger with less body fat up to a certain point. Often very low body fat is associated with lower levels of energy, which can affect strength. Therefore, it comes down to finding out what is the least amount of body fat that will still allow you to still be your strongest.