Carefree or easygoing to the point of being thoughtless; often used with a mildly critical nuance.
彼は能天気だから、心配してもあまり気にしない。
He's carefree, so he doesn't worry much even when others are concerned.
あの能天気な発言は、重要な場では控えるべきだ。
That carefree remark should be avoided in an important setting.
能天気に見えるが、実は深く考えていることもある。
He seems carefree, but sometimes he actually thinks deeply.
能天気な態度が原因で失敗した。
A carefree attitude caused the failure.