I personally feel that the minorness of the lullaby is very appropriate. Lullaby's aren't necessarily defined by a happy tune to make children fall asleep. The Goron Lullaby fits the theme of the atmosphere and circumstances behind making the child fall asleep. The child misses his father, and the song reminds him of his father, because that's the song that was always played to him to get him to fall asleep. Even Darmani fell asleep to that song when he was a child, so it holds tradition and meaning to the Gorons. Simply being in minor would change its significance to their race and to the people that it affects. In a wonderfully musical world, lullabies would be different depending on the atmosphere, because that's what would make the children fall asleep.