Is there an MQTT API call that is equivalent to GET /api/states?
MQTT just shunts messages from publishers to subscribers, so it doesn’t really have a state, other than connected clients and retained messages (and some statistics). So MQTT itself does not have this sort of API - it’s up to the subscribers to store state if needed, and that’s what HA does to facilitiate “devices/states” (for all sensors, including MQTT). What is the underlying thing you’re wanting to do that would be helped by such an API?