'Good at nothing' refers to one's ability. 'Good for nothing.' refers to one's value as a person. They don't mean the same thing.

Hi Mutt, Yes. They pretty much mean the same thing. Worthless. However, in use, I find that people will say "I'm good at nothing", speaking about themselves negatively. And "you're good for nothing", as a cruel statement to someone else. I hope this helps. Thank you, Stephen