Macross Delta was planned to focus on the Walkure idol group, and everything else is tacked on to support Walkure and not to take over the limelight. Mirage in Walkure outfit is a cute crossover/spoof and nothing more. Just like if Minmay is seen dressed up in a flight suit, no one should be expecting the story to take a detour arc with a Minmay as a pilot.
Delta also has too many "important" character. There are 5 singers, 5 pilots on the "good side" and even more pilots on the "bad side". And you have the commander and politicians. You can't expect each character get their own arc of character development. Mirage will never become a primary asset for Delta, she is just a supporting character.
Again, the intend of the Delta is that they want the viewers to like the Walkure, for their songs, for their character. Everything else that's in the series are just support for a flimsy backstory.
BTW, I felt Mirage got a lot of screen time in the movie, maybe not spoken lines. But she was presented as the "other" ace pilot of the final battle. At the risk of claiming it's ripping off Frontier, Mirage reminds me of a lot of Brera Sterne (maybe her ride is purple and all).