I don't think there should be a badge for flagging a post because it can encourage new users to flag just to get the badge.
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One potential problem with flagging badges is that they are awarded immediately. This can make flagging non anonymous in case you cast your first flag on a post and that flag causes a post to disappear.
This happened to me on Area51, a site with relatively low activity...
But it could also encourage people to flag when necessary.
I do believe that people flagging just to get the badge get what they deserve (since a moderator will look at the post and see nothing wrong).