It's not absolutely necessary to dress up. Also know that you can do it if you want and that you don't have to do everything you can or have precision in the period. The most important thing is to be comfortable and dressed for the weather (which, since our fair is held in July, can be HOT). Feel free to go to a Renaissance Fair wearing your normal clothes or buy a ren faire outfit from an online retailer like Grommet's Leathercraft. Or get completely decked out with pirate regalia and so much more. Shops like Grommet's Leathercraft
You can also rent a costume while you're there. The most important thing is to be comfortable and dressed for the weather (which, depending on when the fair is held, can be HOT). If you show up dressed out of context, you can get in touch with some unfriendly attendees at the Renaissance fair who take it very seriously. As an adult with no small children in tow, I was a little disappointed when I visited the Renaissance fair website in my area, and it seemed very, very activity oriented for children.
For help, visit Lost & Found, located in Guest Services within the Renaissance Pleasure Faire. I recently attended the Bristol Renaissance Fair in Kenosha, WI for the billionth time, or so a million. This beautiful white country style dress is a nod to the Renaissance era and the beautiful details and flow of the dress are perfect for a day at the Ren Faire.