The Church has not dealt with this issue in magisterial documents. However, moral theology would seem to offer the following considerations:
Plastic surgery would seem to be warranted if it would provide a significant therapeutic benefit in some regard, either physical (e.g., reconstructive surgery to restore function or utility in cases of accident or birth defect) or psychological. This is provided that the procedure does not damage some other equal or greater good and provided that it is not intrinsically immoral.
Plastic surgery would seem to be permitted—even without significant therapeutic effect—provided that it did not damage a significant good and provided that the procedure is not intrinsically immoral.
Plastic surgery would seem to be impermissible if it damaged a good greater than that to be achieved, being venial if the difference in the goods were light and potentially mortal if the difference in the goods were grave.
Goods that could be damaged by undertaking plastic surgery are varied. They might involve harming oneself—one’s own physical, psychological, or spiritual health—or they might involve harming others, such as being financially unable to provide for one’s family in a proper and timely manner.
As always, if a medical procedure is intrinsically immoral—such as so-called trans-gendering surgeries—it cannot be performed.