Various types of leaf deformation may occur on poinsettia. Often, the damage begins when the leaves are very small. As plants grow, the mature leaves cover the damaged young leaves, so plants are still marketable.
Often, thrips feeding is blamed for distorted leaves. Thrips feeding will result in white scarring as seen in this photo. Poinsettias are not a favored host of thrips, however, they may migrate from spring bedding plants, garden mums or weeds onto poinsettias. Fortunately, this feeding damage is not visible by the time the plants are ready for sale.
Other causes for distorted leaves are environmental stresses, overhead fertilization with phosphorus fertilizers and abrasion.