There are no actual laws against inappropriate dress in Qatar. So the police CAN'T arrest anyone for wearing too little. The whole "You're in a Muslim country, please respect our culture and dress modestly" is not a law it's a suggestion. The most the police could do is suggest to someone that they go home and change.

Malls can put guidelines "No shirt, no shoes, no service" type things, but if they don't enforce it, then you'll still see men and women dressed inappropriately in the malls.