Hot take: Kung Fu Panda 2 is not more serious than Kung Fu Panda, it just dwells into heavier themes.
Rewatching the first movie, I was actually surprised to finally find out that it kind of takes itself pretty seriously? Like, as seriously as the second movie takes itself.
I don't really think Kung Fu Panda started being a more comedic franchise until the third movie, it was just a series of movies with a very silly concept that takes said concept very seriously and uses it to tell a very profound, mature story.
The story of the first Kung Fu Panda movie is about learning to overcome self esteem issues and self hatred, while the story of the second movie is about uncovering themes of genocide, childhood trauma and learning to overcome said tragedy to become the best possible version of yourself, despite all the bad things that have happened.
Each movie takes its themes relatively seriously, while also holding back a lot of the more gory and angsty aspects of said topics.
So no, the second movie isn't more serious than the first one, it just tackles more serious topics.
