This is just my thoughts on it.
I believe the cloud is just for 2 things.
1) To show how large the Pokémon are now. If they dynamax, game freak wants to show that there heads have reached the clouds, they're so big now.
2) (I'm not sure of this) it could be game freak wants to show how good they have become in animation of games, so they are just showing it off.
Hope this helps.