Were all Mexica great astronomers or just the priests?

Good question. Clearly religion, astronomy and astrology were all closely related to each other, and were in turn ‘controlled’ by specialists. Quite how profound a knowledge of astronomical patterns and phenomena was enjoyed by Mexica citizenry in general isn’t really known, since so little on this subject is found in the historical sources - limited basically to a chapter in Sahagún and then to a few sporadic other references.
The economy revolved around rotating solar and lunar calendrical cycles, bound up with the seasons and other natural cycles, so in that sense everyone had an interest in knowing ‘the basics’. As Yólotl González Torres has written, the stars and planets served the Aztecs as ‘compass, map and calendar’ and were central to the farming year. The cycles of the three brightest bodies in the sky - sun, moon and Venus - impacted (and still impact) life of all kinds, human, animal and plant. Thanks to its recurring cycle of birth, growth, fullness, waning and ‘death’, the moon engendered more superstitions and fears than the sun, and was seen to directly influence the coming of rain (the moon was ‘her’self a vessel full of water), plant life, female menstruation, medicine, divination, fertility and reproduction. At the most mundane level, every Mexica knew that the ‘right’ time to cut someone’s hair - and to take sap from a tree - was tied to the phases of the moon...

‘The good (Aztec) farmer’ - one who is ‘careful of things, dedicated, able; a knower of books, a reader of day signs, of the months, of the years.’ Florentine Codex Book X