Correct punctuation: "There's you, off doing good."
It's slang; literally, "There is you off doing good."
More correctly, "There you are, off doing good."
"Off" here means "in some other location or circumstance," similar to "I'm off on vacation."
"There you are" means "I know that it is you from your actions."
What it means is, "This is very typical of you, doing beneficial things in various places."
However, the phrase is sometimes used in a sarcastic or a teasing way, suggesting that you are making trouble or interfering or being inconvenient when you are trying to help.