Obviously it was somewhat based on class, but everyday wear would typically be a white or off-white sleeveless smock or a "pancho" type garment. The garment was usually cottom which they traded for. They would then accessorize with various types of jewelry, depending on the class of individual. From your question, you'd be correct in that the elaborate head dresses and the "show" garments would only be worn during ceremonies. The exception to this would be the emperor, who almost always be dessed "fancy". To their people, the emporers and emprress' were living gods, and thus had to 'play the part'. Hope that helps.
Here's a picture of the garment I was referring to: https://upload.wikimedia.org/wikipedia/commons/thumb/4/47/Vestimenta_Femenina_Mexica.svg/2000px-Vestimenta_Femenina_Mexica.svg.png And here's a good site that has a section on everyday Aztec life: http://mrkash.com/activities/aztecs.html