Is Aristotle's Poetics a critical text? If yes, then what makes it so?
Your question asks one thing, but your tags list a different possible term to apply to Poetics. The short answer to your question is that Poetics is a theoretical text, a text that presents and expands upon a theory about literature, in this case, dramatic literature.
You note "critical theory" in your tag, but ask in your question if Poetics is a "critical text." There is an important distinction to make between a "theoretical" and "critical" text. A critical text would be one that analyzes a work or group of works of literature. Works that analyze plays, for example, would fall under the category of "critical" texts.
With Poetics, Aristotle is developing and explaining a theory about how a play, tragedy or comedy, can be identified. What elements must a tragedy, for example, include? A theoretical text, then, puts forward a concpet or idea that might be used as support for a critical analysis.
To further explicate this distinction, a book like Jan Kott's Shakespeare, Our Contemporary, would qualify as a critical text, as he analyzes characters and plays of Shakespeare, examining their relevance to today's audience. Meanwhile, in order to evaluate the structure of one of Shakespeare's plays, Jan Kott might consult or make reference to Aristotle's theories about drama, as described in Poetics, in order to support his critical analysis.
The links below should help you with further information about Poetics, Jan Kott and literary theory.