It's not a preformed narrative. She sits in a chair during her debut trailer, gets knocked around in two other trailers, and spends the entirety of Kingsglaive as a damsel in distress in need of saving.
This is a selective interpretation of what will amount to ~3% of the game, Kingslaive had shit characterization almost across the board (Noctis >>> Nyx - all the talk about people preferring Nyx is absolutely insane to me), and even there, Luna was among the better characters, damsel in distress or no.
So I wouldn't call this a good grounding for the narrative. The main problem with this debate is that ambiguous words like 'strong' and 'weak' are being thrown around without being given the slightest amount of definition. Does she have to be a good fighter? Does she need to have an active role in the story? Is she allowed to be vulnerable in the face of a vast evil empire? Seriously, this is what gives the "Luna seems weak" narrative a preformed feel: not only is the evidence for it fairly scant, but the metrics people are judging her character against seem completely arbitrary.
Also, goddamn. Since when did Noct have emotions? From being on the verge of falling asleep 24/7 to shouting 70% of the time he speaks.
Loved the trailer, though.