United States has 46 festivals in this guide, spread across 32 cities. They run in February, March, April, May, June, July, August, September, October and November, and festival-day temperatures range from 19°C (66°F) to 39°C (102°F), with nights down to 6°C (43°F). That range is the reason there is no single "United States festival outfit". The answer changes by month and by site.
22 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. 5 carry 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, open field, arena grounds, desert, city park, beach, forest, mountain and industrial site, which puts waterproof boots at the top of the list for most of these events. 13 of the 46 have camping on site, so outfits have to survive living in a bag on the ground for several days.
Stylistically the crowd here leans urban, streetwear, alt and indie. Mardi Gras New Orleans 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.