system
Given the following post, respond as if you're commenting on Reddit:
human
[Super Mario/Smash Bros.] Why is Bowser more animal-like in Smash?

I mean, he seems perfectly intelligent in the Mario-verse, what with the plans he pulls off and the ability to speak. So why does he only communicate with roars and grunts in Smash?

Additionally, how come Bowser Jr. and the Koopalings are perfectly normal?
gpt
Based on what we see it appears that the whole Smash Bros Universe is inside the imagination of a child.  That explains why heroes and villains from wildly different universes all fight one another.  And this explains why relatively weak characters like the Animal Crossing Villager or Little Mac can compete against literal gods like Palutena.  The more bestial form of Bowser is what the child believes Bowser is like.  While, perhaps since they are closer in age to the child, he/she perceives Bowser Jr. more accurately.

There is also a theory as to why Bowser appears so different in different games, and also why he is allowed play games and race go-karts with his arch-enemy and frequent kid-nap victim.  In the Super Mario Bros 3 there is a clear theatrical motif.  The background appears to be that of a theater back-drop.  This has lead to speculation that the entire Mario Bros universe is a show designed to entertain.  Every version and form of Mario and Bowser are just different incarnations of the same entertainment franchise.  When they go-kart or play tennis it is just the actors having fun and playing with each other as friends.