I do agree that some expat girls are giving everybody else a bad impression. Yesterday, whilst having coffee at the Souq my friends and I were dismayed to see two expat women wearing skirts way too short, drawing extra attention from the public. It was crass and unnecessary.
I can see the need to respect the sensitivities of local women and their culture. After all, we are guests and must respect our hosts. However, to find modest clothes in Doha is a tricky business, especially if choosing clothes from big Malls such as Villaggio or Landmark. We are faced with overly exposed Summer clothing or middle aged size 20 plus :(