If I want to say I am good at something but not very good can I say "I am about good" does this phrase considered slang.
This wouldn't be standard, and I wouldn't use it formally. However, in response to the question "Are you good?", you could say "I am about good", with a lot of emphasis on the word about. However, "about" is an unnatural modifier for "good", because it "about" usually refers to a specific quantity, time, or place — which "good" isn't.
In American English, I think the normal response, if you want to say that you're good, but not perfect, would be "I am pretty good."