It's an idiom meaning mediocre, leaning a bit more to the positive. On a scale of 1-10, about a six or seven.
What is the meaning of halfway decent? Can anyone explain?
When someone says something is "halfway decent," they mean it's not bad, but it could be better.
It's an understated compliment similar to "that's not half bad". It's an idio I'd expect to hear in Britain rather than the US.