Mexico has 8 festivals in this guide, spread across 4 cities. They run in January, February, March, April, August and November, and festival-day temperatures range from 22°C (72°F) to 30°C (86°F), with nights down to 8°C (46°F). That range is the reason there is no single "Mexico festival outfit". The answer changes by month and by site.
3 of them run in real heat (28°C or more), where loose natural fabrics beat coverage and a brimmed hat does more than any garment.
The ground matters as much as the sky. You will find forest, arena grounds, city streets and beach, which puts flat sandals you can rinse, plus trainers for the night stages at the top of the list for most of these events. 1 of the 8 has camping on site, so outfits have to survive living in a bag on the ground for several days.
Stylistically the crowd here leans tropical, boho, costume and streetwear. EDC Mexico is the largest of them at around 100,000 people, and it sets the tone the smaller ones react to. None of that is a dress code. It is what you will see, which is worth knowing if you would rather blend in or deliberately not.