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.