(Information correct as of May 18th 2020)
You mother can enter the US since she is a US citizen.
From this Whitehouse article:
I have determined that it is in the interests of the United States to take action to restrict and suspend the entry into the United States, as immigrants or nonimmigrants, of all aliens who were physically present within Iran during the 14-day period preceding their entry or attempted entry into the United States.
As of yet, there is no mandatory quarantine but it is highly recommended and health checks at OCD are in place.
OCD's website states:
The Chicago Department of Aviation (CDA) has no higher priority than the safety and security of the traveling public and our airport community, and we are taking every precaution to keep travelers and employees safe. We are in close contact with our federal and public health partners at the Centers for Disease Control and Prevention (CDC) and the Chicago Department of Public Health (CDPH), and strictly adhere to their recommendations. We are determined to maintaining normal airport operations, while at the same time supporting CDPH's mandate to reduce the impact of COVID-19 in Chicago.
CDC's website states:
At this time, travel restrictions and entry screening apply only to travelers arriving from some countries or regions with widespread ongoing transmission of COVID-19 (Iran is one of these countries). [Note: US policies are subject to change as the COVID-19 pandemic evolves.]
You may be screened when you arrive in the United States. After you arrive home, take the following steps to protect yourself and others:
- Stay at home and avoid contact with others. Do not go to work or school.
- Monitor your health. Take your temperature with a thermometer two times a day and monitor for fever. Also watch for cough or trouble breathing.
- Keep your distance from others (about 6 feet or 2 meters) This is referred to as “social distancing.”