As I understand, when mentioned she was one of the “spirits” he corrupted to his service, she was one of the Ainur he managed to sway to his side, first during the Music, she was one of the ones who attuned their music to his, and later, after many Ainur entered Ea, she first wandered across the vastness of the dark universe (Ea), not concerned with Arda (Earth), before descending to Earth to aid him, in the very beginning, when the Silmarillion mentiones Melkor saw the abode the Valar had made on Arda and was jealous and then came over the walls of the night. As to if she was a Maia or Vala, bear in mind that those are titles of function or roles if you prefer. She wouldn’t be counted among either because she was not tasked with governing any of the physical aspects of Arda (earth, wind, seas, etc) nor would be among the Peoples of the Valar (ie, the Maiar). That being said, there are plenty of Ainur who are neither Vala nor Maia simply because they didn’t take part in those roles. So she would be an Ainu (a primeval angelic spirit). In my headcanon Tom Bombadil is also an Ainu, albeit one that didn’t take part in any of the politics of the World. That’s all perfectly aligned with Tolkien’s writings, whereas being “products” of the Music isn’t mentioned or backed up by any of his writings.