I know it can be so hot when it comes to being a guest at an outdoor wedding, but it is still important to always dress appropriately.
Now, as a planner, I am always in black so they know I am working the wedding. But as for a guest, it is important to be wearing something comfortable, but also nice. A summer dress is always a good idea if it is also the appropriate length. If it was your wedding, you would want your guests to dress appropriately as well. It is so important that you listen to what the invitation is telling you about what to wear when it comes to any type of wedding.
I always say it is better to be a little overdressed than underdressed. It is okay to be known as the one who likes to dress up than the one that is known for wearing jeans to a wedding if that was not the dress code.