In one of my robots we use 2 mice mounted underneath which basically act as encoders for the wheels. They work pretty well when mounted intelligently and if the mouse likes the surface.
Unless your mouse was oriented in the same direction all the time (i.e. it didn't turn when the robot did) you wouldn't get an X-Y location (assuming your robot has the common 2 or 4 wheel configuration). The mouse can't detect turning so it would only report movement in 1 direction.
Give us a little bit more information and we might be able to help further