Is ''Whom'' a predicate nominative

No, apredicate nominative

is the noun or a pronoun following a linking verb that restates the subject.

The Word 'whom' is the only objective interrogative or relative pronoun, a Word that introduces a question or a relative clause. The pronoun 'whom' is always the object of a preposition, not the object of a verb.

interrogative pronoun: To whom do I give my completed application form (object of the preposition 'to')

relative pronoun: The person to whom you give theapplicationis the manager.