The problem is I feel this humor may have come partly to cope with the loss of her sister so many years ago, and that perhaps it was Celestia that setup the protocol we were seeing Luna using during Nightmare Night over the years she has changed, relax this protocol, adapted perhaps purposely so that she could be closer to her subjects. Yet at the same time I do not quite think the Element of Laughter fits Celestia per say, though she does have a sense of humor. She she was shown to be very serious, not even understanding what fun was and perhaps she didn't understand what it was until then. On Luna and what elements she would be the wield I feel we cannot look simply at the episodes she's currently been in as it is very likely much is changed since she had become Nightmare Moon. This element is not based on how powerful the magics of the wielder but instead represents a form of unity, in this case that a friendship, the magic of the bond that can be formed between ponies so different from each other. Now before I go on I would like to express my definition of the Element of Magic. Honesty is a much more fitting element for Luna as she seems to care quite a bit about honesty and is shown to be very honest with how she feels and what she thinks. I would not have Celestia wield the Element of Honesty because of the fact she has from what I've seen not been the type to always be fully honest, perhaps even to herself. This can be partly why she lost her connection to the elements she did wield because she had not show such care to her sister before she turned into Nightmare Moon, her time and attention.

This could have also gone into what the issue was as Celestia could have gotten so absorbed in her responsibilities and duties that she would leave very little time if any for her sister, even to notice that Luna was struggling. My reasoning for this is because of how deeply Celestia has been shown to care about her subjects and how I would see her being with type to give all her time and attention to helping her little ponies even back then.

I personally feel that Celestia wielded the Elements of Kindness and Generosity, as well as possibly the Element of Magic. Which Elements of Harmony did Princess Celestia and Princess Luna wield so many years ago?

Celestia and luna mlp