I am of the opinion that it is not, per the following line of argument:
The definition of "deus ex machina" (according to dictionary.com) is: "any artificial or improbable device resolving the difficulties of a plot".
P1) Aang, as the Avatar, is closely tied to the spiritual-moral fabric of the Avatar universe.
P2) The moral duty to take the Fire Lord out of power is something directly imposed upon him by this role.
P3) To go against his conscience and kill someone upon pre-meditation would be to self-inflict a very deep scar upon himself, corrupting him for life, which in turn would not bode well for the Avatar universe via their close connection.
P4) It is therefore a natural part of the Avatar universe to correct for this spiritual instability (something the Lion Turtle would be sensitive to, due to its apparently deeply meditative approach to the cosmos) by offering a practical way out of the need to kill the Firelord upon pre-meditation (which seems as if it would always be equivalent to cold-blooded murder in Aang's spiritual view), if he absolutely needed it.
P5) This practical way (that is, Energybending) out consists of requiring Aang to confront the problem directly; that is, the practical way out required him to master the possibilty of being corrupted (in general) due to direct contact with corruption in others. (Learning it via the Lion-Turtle only makes sense, as dealing with the nature of spiritual disruptions in the universe seems to be an area which the creature would have special knowledge of, due to the aforementioned nature of the beast.)
C1) The Lion-Turtle and its teaching of Energybending to Aang was neither improbable, nor a resolution to the problem completed from tthe outside (as it were) upon which Aang's subsquent actions had no bearing (and therefore cannot be labeled as artificial, in that sense). Therefore, they cannot be properly referred to as examples of the plot device labeled "deus ex machina".
Please comment on this blog, and explain your own views on the subject.