Thailand has 3 festivals in this guide, spread across 3 cities. They run in January, April and December, and festival-day temperatures range from 30°C (86°F) to 36°C (97°F), with nights down to 22°C (72°F). That range is the reason there is no single "Thailand 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.
The ground matters as much as the sky. You will find beach, city streets and open field, 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 3 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, party, costume and beach. Songkran Bangkok is the largest of them at around 1,000,000 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.