what kinds of imperfection does the birthmark represent

The birthmark symbolizes “the fatal flaw of humanity,” it also symbolizes mortality by extension (153). The birthmark is an implicit reminder of every human’s limited lifespan, which is imposed by the incomprehensible forces of Nature. The birthmark, therefore, functions as a source of anxiety for Aylmer (who fears death and its equal degradation of all humans) because almost-perfect humans like Georgiana will endure the same fate as everyone else.