Graphics are not really mirrored, or is it just me?

No need, maybe someone else with this problem will find it. =) Glad you figured it out! I thought there was a checkbox to toggle that behavior, but I didn't want to comment until I had a chance to check.

As I recall, the reason for this is that before Robocode itself had graphical debugging, someone made something called "Robocode SG" to enable it, and the API adopted by Robocode is the same except for fixing the Y coordinate orientation.