No Flagged email folder on iCloud browser?
There is no Flagged mailbox in the Mail "app" of iCloud.com. It also does not appear after flagging a message.

At present, you can:


use Mail.app (or another computer mail client)
use an iOS device to see the flagged mailbox that was introduced in iOS 6 for iOS devices.
ask Apple to add this to iCloud at http://apple.com/feedback