When you do a good job and Japanese person says baka janai does it mean not stupid after all?

The Japanese phrase 'baka janai' does not mean 'not stupid' or anything positive after all. 'Baka' means 'fool' or 'stupid' and 'janai' is the negative form of 'to be', so 'baka janai' means 'you're not a fool'. However, it is often used in a sarcastic or condescending manner, so when a Japanese person says it to you, it is more likely to mean 'you're an idiot' or 'don't be ridiculous'.