joyous; auspicious; happy — used for celebratory or felicitous occasions.
赤ちゃんが生まれて本当にめでたい。
It's truly wonderful that a baby was born.
結婚は家族にとってめでたい出来事だ。
Marriage is a joyous event for the family.
新しい門出を祝って、皆でめでたい会を開いた。
To celebrate the new start, everyone held a joyous gathering.
naive; simple-minded — used (often negatively) to describe someone as gullible or blissfully unaware.
彼は危険なことをするのに平気で、少しめでたいところがある。
He's unbothered by risky things and is a bit naive.
そんな甘い考えでは通用しないよ。おめでたいね。
That naive thinking won't work. How naive.