Raven Queen is not a Forgotten Realms Deity. She is just some being from the Shadowfell.
That’s a reason for her having Warlocks and not clerics. It’s not unknown that a far realm being having warlocks to further their influence beyond their realm. I can see (not really the Raven Queen), a deity from another plane sign a contract with a Warlock to set up a following and/or build a temple to start up a fledgling religious order.
