I think it's kind of lame to control the mech's cockpit with the mouse.
I think I've figured out a way to solve this problem:
You rotate the cockpit as if you were driving a bulldozer or similar construction machinery. You need to rotate it independently with other controls, and the cockpit doesn't rotate smoothly, but like machinery.
As for the cannons, you control them independently with the mouse, within a certain limited area. You just aim with the mouse on the screen and the Mech's cannons align automatically. If the enemy is outside this limited area, you need to correct the Mech's cockpit so that it enters the area.
I think it would be much better. Mech games where the cockpit just follows the mouse lose that sense that you're operating a huge fucking machine.