On the one hand, you can certainly argue that the Spanish won and the Indians lost when the Spanish conquered the Aztecs. The Spanish ended up taking power and becoming the elite of Mexican society. The government leaders were Spanish. The Spanish religion was the religion of the country. The gold and silver from the mines of Mexico went to make Spain rich. The Aztec government and religion were destroyed. In that sense, it is easy to say that it was a victory for the Spanish and a defeat for the Aztecs.
On the other hand, we can say that the Spanish did not truly win and the Aztecs did not truly lose. This is because the culture and the people of Mexico essentially became a mixture of Spanish and native cultures. While Catholicism became the national religion, it was influenced by native beliefs. The Day of the Dead is celebrated and the brown-skinned Virgin of Guadalupe is the major saint of Mexico. While the Spanish took control of the government, the society as a whole became a mix, with mestizos becoming dominant. Not long after the conquest, the whole society had become mixed, meaning that the Spanish did not truly win and the Indians did not truly lose. Which of these arguments is more compelling to you?