I don’t have a problem with this. Just as when tourists visit St. Peter’s Basilica, they should adhere to the dress code there.
My question is if they are visiting a mosque, are they also visiting a church, synagogue, Buddhist temple, or any other house of worship? And if not, why not?
I have no issue with the dress code. People should dress appropriately in church. Women wear headscarves at our church (Russian Orthodox) though sometimes among younger women, the hemlines can be a little high and the shoulders might not be covered. And then the older Russian ladies give them dirty looks!