How is the creature a tragic hero?

The answer to your question greatly depends upon whose
definition of a tragic hero you adhere to.

According to
Aristotle, a tragic hero must possess the following ideals/characteristics: noble statue
and possess greatness, occupy a high position and embody virtue, be considered great
(but not perfect), the downfall is the fault of the hero, misfortune is not completely
brought about by their own actions, their fall increases self-discovery, the hero's fall
does not leave the audience (or reader)
depressed.

According to Shakespeare, (in addition to
adherence to Aristotle's characteristics) a tragic hero must possess the following
ideals/characteristics: downfall due to pride, doomed from the start, typically a leader
(or king), suffering must be for a reason, tragic hero is typically
male.

Therefore, based upon these characteristics defined
by both Aristotle and Shakespeare, Frankenstein's monster would not be considered a
tragic hero if all of the characteristics are used to define him as
such.

On the other hand, the monster does possess some of
the characteristics which define a tragic hero: misfortune is not brought on solely by
the monster (Victor is responsible), he is doomed from the start, and is suffering for a
reason.