I want to buy a cat litter box for my girlfriends parents, and then say things about how bad the situation was with the cat.
Don't you think that's a little rude?
They are not kind to me. It's obvious that they're brain lazy, because the cat always makes a mess on their floor
Buying a litter box for your gf's parents just to point out the fact that the cat goes to the bathroom on the floor is rude and manipulative. If you have a problem with the cat, deal with then directly.
They don't care, they love the stench. I saw a bottle of perfume on her mom's dresser called, Bouquet d'urine Feline.
Using your approach, all you're going to do is make them think you're a loutish buffoon. If you don't want to be direct with her parents, ask your girlfriend to talk to them.