Dia de los Muertos, y los Inocentes, y Angelitos are holidays that were more recently pagan (Aztec) that have been adopted by the local people who converted, and have spread (Araw ng mga Patay in the Phillippines) to some places but not others. It was not an active church policy to create this celebration, but more of something that happened. You can say they shouldn’t let it happen, but to say that the RCC specifically created these combinations is silly. The Day of the Dead (Dia de los Muertos), is not big in the United States except in areas with a large Mexican population (Arizona, California, and Texas), if you were correct and the church was actively promoting aspects of paganism then Irish Catholic kids in Massachusetts would also be instructed in the Day of the Dead, and it would be another holiday here in the USA. Just as people in Honduras would celebrate St. Patrick's day with a big parade and a day of fun in the middle of Lent, but that is not the case, is it?
More's the point, many of the cults, saints and celebrations in Mexican Catholicism are specifically condemned by Rome as non-Christian, but a community's folk beliefs evolve on their own, regardless of what someone in a position of authority may have to say on the matter.