The term "antihero" normally refers to the protagonist of a literary work. The typical heroic protagonist is portrayed as a larger-than-life figure with many admirable qualities. In traditional heroic epic, the hero is an aristocrat who is strong, physically attractive, and morally good and who, through his virtues, triumphs over various obstacles. The antihero is normally a protagonist who does not conform to the conventions of heroic epic. The antihero can be a despicable character, as the narrator of Dostoevsky's Notes from Underground, or sometimes simply a realistic and fallible person, such as Holden Caulfield in The Catcher in the Rye.
Because Frankenstein's monster is not the protagonist of the novel, he is not normally considered an antihero. Instead, Victor Frankenstein, as a flawed protagonist, is closer to that role. Although Victor has many of the external attributes of the hero in that he is a handsome, wealthy, intelligent aristocrat, his moral failings make him in certain ways almost more monstrous than his creation.
The monster plays the role of an antagonist in the novel, not a hero. However, just as Victor is a flawed and thus unconventional protagonist somewhere between hero and antihero, similarly, the monster is not really a traditional villain and is portrayed sympathetically, more as a victim than as a force of evil. The monster obviously lacks the characteristics of a hero, being physically repulsive, not of noble lineage, and not a model for emulation, but because he is not the protagonist of the novel, he might better be described as a sort of "anti-villain" than as an antihero. Readers are led to pity rather than revile him.