An Inappropriate Username can be reported mid-match by selecting the Opponent's Avatar. An Opponent's Profile screen will appear with a Report Button located in the lower Right of the Popup. You can tap this Button and fill out a prompt for the Inappropriate Username.

This can also be done from the Friend Menu by selecting the Username from the List of recently played Players. Tapping it will open up the Opponent's Profile along with a Report Button. This Button can also be tapped to bring up a Prompt to Report the Username.