A witty response to "I don't give a fuck" or "I don't care" can add a touch of humor while also asserting your confidence. Here are a few options:
"Well, don't worry, they sell care packages at the store if you change your mind."
"Ah, the care-o-meter is off the charts!"
"Congratulations, you've reached the peak of apathy! A medal is on its way."
"That's the spirit! Care-free and living the dream."
"It's okay, caring can be overrated. More caring for me!"
"And the 'Caring Award' goes to... everyone else!"
Remember, wit should be used with care and consideration of the context and the relationship you have with the person you're responding to. It's all about keeping things light and fun without causing offense or escalation.