Let me clear this up. Mathieu plays corner when we are in a base 4-3 defense, but he is not our "shut down" corner. I believe that Morris Claiborne will be lined up on Rodgers more than Mathieu.
When we are in a nickel formation, Mathieu plays the nickel spot. Tharold Simon comes in and plays the corner opposite of Claiborne.
Mathieu does a lot of different things though. He blitzes a lot for a corner or nickelback. He plays man coverage against a wr, te, rb all on different plays. He can drop into deep coverage, intermediate or play near the line.