Chile has 1 festival in this guide, spread across 1 city. They run in March, and festival-day temperatures range from 29°C (84°F) to 29°C (84°F), with nights down to 14°C (57°F). That range is the reason there is no single "Chile festival outfit". The answer changes by month and by site.
Every one of them runs 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. Every festival here is on city park, which puts broken-in trainers or a low boot at the top of the list for most of these events. None of them have camping on site, so you are travelling in and out each day and the outfit has to survive the journey at both ends.
Stylistically the crowd here leans y2k, streetwear, indie and pop. Lollapalooza Chile 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.