Wiktionary
not bad
a. (context idiomatic English) reasonably good.
WordNet
Usage examples of "not bad".
If you are going to say, as theologians do, that God is good, you must then say that right and wrong have some meaning which is independent of God's fiat, because God's fiats are good and not bad independently of the fact that he made them.
I finished in 19th place, not bad for a guy recovering from cancer, but I didn't see it that way.
Good lips, she thought, really not bad lips at all, and a thin boy in a white shirt and dark leather jacket smiled at her from the up escalator, a huge black portfolio case beneath his arm.