I haven’t really thought about TLJ through the lens of postmodernism, but it makes a lot of sense. It seems to me that the primary aspect of postmodernism is a rejection of established ideology and absolute morality/truth. In this way, TLJ is absolutely postmodernist and it doesn’t really matter if Luke ends up affirming the Jedi myth or not in the end - this critical attitude permeates every aspect of the story.
Keep in mind that Yoda, the preeminent authority on the Force in this universe, dismisses the foundational texts for his religion as ultimately irrelevant when urging Luke to take action. Luke is not being castigated for failing to follow the Jedi way - instead both he and Yoda seem to be in agreement that the Jedi as an institution and set of doctrines no longer serves a purpose except as a way of inciting the masses. Nevertheless, Luke decides to reaffirm the authority of the Jedi while not refuting this critique.
The Finn/Rose storyline introduces a character whose sole purpose is to question the villainy of the First Order and the heroism of the Resistance, and his character is rewarded for doing so - and he’s the only character of this storyline who materially profits from it. Nevertheless, Finn proudly proclaims his loyalty to the Resistance and is ready to give his life for this institution, even knowing that it may be built on a corrupt financial foundation.
The Poe/Holdo storyline does its best to criticize the very idea of leadership, as it dispenses with all the established leaders of the Resistance and replaces them with one character who goes to cartoonish lengths to demonstrate her ineptitude. Nevertheless, Poe comes to accept her leadership after learning of her plan, and returns to his role as a leader himself within the Resistance.
The Snoke/Kylo storyline is the epitome of the postmodernist take of course, with Snoke’s unceremonious departure illustrating a complete disdain for both established authority and grand theories of its reason for being. Kylo is the only character in this story who accepts the postmodern take and runs with it.