I travelled to the countryside with a Mongolian girl and we both wore jeans. It is fairly conservative outside of the capital but the weather is the biggest factor. I recommend jeans, short sleeve tee shirts and a long sleeve shirt to wear over it. Most women wear a headscarf to protect from the VERY strong sun – I burned faster in northern Mongolia than I did on a beach in Thailand. Even in June we both wore a sweatshirt at night as the temperature changes very much. Pack nothing fancy and nothing that cannot be washed easily since even UB is very dusty and outside of that there are few paved roads.
Shelley, Tampa, USA