Here's a scenario. Hamza says: "Mary, I fixed your car. I hope you have a good day." Mary says: "It [my day] will be better than it would have been [had you not fixed my car].

Hello Hamza, Is the sentence "It will be better than it would have been" ?? That one doesn't make any sense.

Do you mean, 'It would have been better'? For example, It would have been better if you had called an ambulance as soon as possible.