Indonesia has 2 festivals in this guide, spread across 1 city. They run in July and December, and festival-day temperatures range from 31°C (88°F) to 32°C (90°F), with nights down to 24°C (75°F). That range is the reason there is no single "Indonesia festival outfit". The answer changes by month and by site.
Every one 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. 1 carries a high rain risk, which makes waterproof footwear a requirement rather than a precaution.
The ground matters as much as the sky. You will find city streets and industrial site, 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 streetwear, y2k, kawaii and urban. Djakarta Warehouse Project is the largest of them at around 90,000 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.