I don't think the mouse was programmed when using DOSbox. I don't know how to fix this. in fact when I used to play Mechwarrior 2 for PC I only used the keyboard. Remember this was an early 1990's game and not many people that I am aware of used the mouse for aiming back in the day. I'd say you could try to run it with windows but come to think, I never used the mouse in that game other then to start the game up
to rotate the torso I pressed the greater than or less then < or > key left for left and right for right.
Press any one of the arrow keys to turn your entire mech and 1-0 on the keyboard to walk forward
I would also read the key configuration because this game has a lot of controls