I try not to nerf character classes for balance reasons because to be honest, once you start where do you stop!?
That said, it has been suggested that nerfing is done to make things easier for the Dm or less fun for the player. To be honest if I ban/nerf something it’s almost always because it either shuts down an element of the game that could be fun (alertness, lucky, rope trick) or because it annoys other players (moon Druid).
It may be great fun for the Moon Druid to transform into the most optimal creature for its level - bear for instance. But I have personally dealt with the complaints from other PCs who feel overshadowed and superfluous because of how OP it is at that level. I’ve found that telling players it gets better in 5 levels provides very little consolation.
I’ve seen some Moon Druids use their ability sparingly though and not have this effect. I’ve seen others where it is their go to tactic and it becomes a problem. I’d say the same about the Twilight cleric.
The question is not does it make the DMs game more fun, but does it raise the overall fun of the table (of which the DM is a component)