Is there a difference between the actions of a legally competent person in jest and in seriousness, and is joking considered less sinful when committing acts of disobedience such as cursing, backbiting, and slandering?

There is no difference between seriousness and jest when it comes to prohibitions and the rights of Allah. Divorce, marriage, and revocation (of divorce) are valid even if uttered in jest, by consensus, based on the Prophet's (peace be upon him) saying: "There are three matters, the jest of which is serious, and the seriousness of which is serious: divorce, marriage, and revocation (of divorce)." Disbelief (kufr) also occurs through jest. One who jests is not excused, unlike one who is compelled, mistaken, or forgetful. Some scholars differentiate between marriage and similar contracts that involve a right of Allah, and transactions like sales, which are purely financial dispositions. They hold that jest is binding in the former but not the latter, reasoning that the jester intended the utterance, and the Lawgiver intended the ruling to apply to him.