You can wear a dress and not be dressy.
In other words, it is acceptable to wear a dress even if you're not trying to look fancy. Quite the contrary really. Dresses today (as opposed to back in MY day) are made of cotton and feel like tee shirts. It's phenomenal really. Now that it's chilly, just throw on a pair of nubby tights or leggings, throw on some boots and you're good to go.
The more you know!