They sure do: https://www.eda.admin.ch/tehran
But that would be Swiss soil so the Iranians probably don’t care if the Swiss fly a flag emblazoned with a cross on that there.
That’s a view of the front entrance to their embassy. No flag is visible, for what it’s worth. I’m going to look for photos of the Scandinavian embassies.
It looks like the Swedish embassy displays its flag (scroll through photos).
https://www.google.com/search?q=sweden+embassy+in+tehran&oq=sweden+emb&aqs=chrome.2.69i57j0l3.11761j0j4&client=ms-android-verizon&sourceid=chrome-mobile&ie=UTF-8#fpstate=lie&xxri=28