The mirrors were devotional objects in the indigenous religion of the Nahuas, analogous to Christian crucifixes. They were symbols of divine power. The Catholic Church to this day ascribes supernatural power to physical objects such as the Eucharist, and 16th-century Spaniards certainly would have believed that the relics of saints conveyed divine benefits. The Church itself reaffirmed the miraculous power of relics that same century at the Council of Trent. There is really no reason to consider these Nahua religious objects "occult" except by a definition that includes an exception for Christianity.
In any case, the conquerors of New Spain did not encourage the Nahuas to reform their indigenous religion on a rational basis free of superstition. They did not train up an Aquinas for Tezcatlipoca. They suppressed the religion of their conquered subjects in favor of their own, a tale as old as written history.
https://www.newadvent.org/cathen/12734a.htm
In any case, the conquerors of New Spain did not encourage the Nahuas to reform their indigenous religion on a rational basis free of superstition. They did not train up an Aquinas for Tezcatlipoca. They suppressed the religion of their conquered subjects in favor of their own, a tale as old as written history.