To start with, the two holidays fall on separate days of the calendar. Day of the Dead is a two-day festival that stretches from midnight on November 1 to just before midnight on the 2nd (via History). Dressing as a Day of the Dead figure on October 31 is like wearing a Santa Claus suit to your Thanksgiving dinner. (Isn't it bad enough that stores put out their Christmas trees right after Halloween?) But more significantly, wearing a Catrina outfit could be considered cultural appropriation if it's not part of your own identity.
Día de los Muertos is a tradition dating back to the ancient Aztecs, who believed that souls traveled on a nine-step journey to their final resting place. Family members would hold ceremonies in which they would offer food, drink and other items to help the departed along their way. Even now, the holiday is meant to be a time to remember and celebrate loved ones who have died. The traditional painted sugar skulls are meant to represent the deceased, as well as the importance of recognizing the "sweetness of life" (via Smithsonian).
Day of the Dead is definitely not "the Mexican Halloween," as some put it, per USA Today. And though it may have been acceptable years ago to wear a Catrina or similar outfit for trick-or-treating, it's thought to be offensive today.
