Is necrophilia considered a mental disorder?

Yes: Necrophilia (the erotic attachment to corpses) is a paraphilia. Paraphilias are recurrent, intense sexually arousing fantasies, sexual urges, or behaviors generally involving nonhuman objects, the suffering or humiliation of oneself or one's partner, or children or other non-consenting persons -- occurring over at least 6 months. For some, sexual arousal is possible only with the preferred fantasy.

Paraphilia: Paraphilia is an umbrella term for less common sexually-related mental health conditions. Necrophilia is considered a paraphilia similar to the way pedophilia is a paraphilia. Yes, necrophilia is defined as a mental disorder.