Sugar is neutral in PH, but can be used to help balance out overly acidic food. I would be worried that in this case it would just bolster the flavor of the lemon if it is strong enough that you can detect it in the sauce.

You could try cutting it with some fat (milk, cream or butter), and I would try adding more onion and/or garlic.

You should probably consider buying a different base tomato sauce in the future.