Works of imaginative literature are conventionally divided into poetry, fiction, and drama. Poetry is written in verse. Although some dramas are composed in verse, such as many of the plays of Sophocles and Shakespeare, Top Girls is written in prose, and therefore is not poetry (nor even poetic or verse drama).
The remaining two genres of imaginative literature are distinguished by what Aristotle defined as the modes of imitation. One possible form of imitation is "mimesis," in which the work takes the form of dialogue involving the characters and in which actors can pretend to be the characters on stage; this is called "drama." The other mode is "diegesis" or narration, in which a narrator tells a story describing characters; this is "fiction."
Top Girls is a play or drama because it consists of characters speaking their stories and it was specifically written to be performed by actors on stage. There are embedded narratives within the play, but the overall structure is dramatic.