Kakarot is Goku from an alternative timeline, where he never hit his head and became to fun-loving and charming Goku you know today. Instead he grew to be the aggressive and cold saiyan warrior he was supposed to be, and ended up conquering the planet, along with his prince Vegeta, his suboordinate Nappa, and his older brother Raditz. He may not be anywhere near as strong as his original counterpart, but his strength and abilities are still by all means nothing to scoff at.