I imagine it went something like
"We want Marth in Awakening, but we can't do new animations and models."
"Use Lucina without a mask."
"But he doesn't wear Lucina's clothing."
"Then MAKE him wear it."

Marth in Awakening's DLC is a headswap of Lucina. All of the DLC characters got new artwork depicting them as existing classes in Awakening, so that the developers wouldn't have to actually make new models for them. EFFORT.