This series of posts, in this very subreddit, will probably help you a lot. But one of the most relevant posts there, from /u/Mictlantecuhtli

So, the general Maya population probably could not read the inscriptions. And the writing was not always seen by the general population. Even things like stela or carved lintels were in elite areas and most likely seen by only the elite. Your average Maya peasant was probably not hanging around the temples or palace structures. Even courtyard areas where people would gather were probably still restricted to the elite. If you look at elite buildings, especially palace groups, the rooms a quite small. This is done for a reason. Inside is where the ruler sits, is surrounded by draped cloth and luxurious groups. Anyone below him had to stand outside and look in and sometimes up. This gave the ruler a physical position of authority. If you have carved lintels or elaborate stucco sculptures around that palace room it only adds to the ruler's status and prestige including any writing that shows off his accomplishments. Most of the stela covers three main parts. The birth of the ruler, the accomplishments of the ruler, and the death of the ruler. The accomplishments include enemies captured and sacrificed, cities captured, when they ascended the throne, when their children were born, etc. It was very much propaganda for the powerful. "Hey, look how awesome I am. You should continue to support me or I may have to hurt you." kind of thing.

As far as I know the Aztecs did not have full writing like the Maya. Only proto-writing. They could express time and date, express titles by combining the associated sounds of pictograms together (shield/chimal-li + smoke/popoca = Emperor Chimalpopoca), and had common ideograms, but they could not express a full grammatical sentence. So the entire civilization would be functionally illiterate.