You may think his mom sucks, but you need to show respect, and keep your feelings aside. If he had issues with your parents, you’d expect him to act mature and keep his feelings aside as well.
Or you can continue mouthing his mom in discussions, and make him feel worse. At some point, he’ll get sick of you and throw you out. Any Pakistani man would.
But then, you probably would be cheering on your brother if your sister-in-law did the same with your parents.
So just treat others how you want to be treated. You don’t need to love her. Just pay your respects and maintain your distance. Let your husband know that this is your stance, and he’ll probably be ok with it. As long as your mouth is not running, most men wont care if you want to make their moms your best friend or not.