The poem has characteristics of a simple ballad, particularly the first four lines of the poem, which follow the abab format and the lines have alternating three and four stress lines (called "ballad meter").
It has a sing-song quality to it and tells a story -- two characteristics of any ballad. The rhyming scheme is fairly simple and repeated as well.
In the fifth stanza, Poe breaks out of the 3/4 foot lines, when he adds an extra three-foot line:
“Of those who were older than we—
Of many far wiser than we—.”
Given these characteristics, I think you could safely say it could be considered a ballad.
Actually, “Annabel Lee” is an elegy in memoriam of Annabel Lee, but it is also a famous example of romantic poetry because it is a piece of literature written by the poet in meter or verse expressing various emotions by the use of a variety of techniques including metaphors, similes and onomatopoeia. Emphasis is on the aesthetics of language and the use of techniques such as repetition, meter and rhyme are what are commonly used to distinguish Romantic poetry from Romantic prose.
I'm currently taking a college course on Edgar Allan Poe and my professor said that "Annabelle Lee" is a lyric poem. He also says that it is Poe's best lyric poem.
I daresay I see no onomatopoeias in the poem, there were metaphors but I think it could be a ballad and romantic poetry although I could be mistaken. There are a lot of characteristics that would define it as romantic but the way the syllables are much like a rhyme or childrens stories would make me think of it as a ballad.