Is whom a relative pronoun?

Yes, the pronoun 'whom' is a relative pronoun. The pronoun 'whom' is also an interrogative pronoun.

The pronoun 'whom' is the only objective relative and interrogative pronoun, which normally functions as the object of a preposition.

Examples:

The customer for whom we made the special cakewill pick it up at four. (relative pronoun)

To whom do I give my completed application form? (interrogative pronoun)