I feel the use of the Victory Pose in the Generation 7 and 8 games are used to make the character more realistic and as an easter egg. There weren't a lot of facial expressions but the pose could be a symbol. A symbol which foreshadows their victory in the Pokémon League. Furthermore, this could also reference the Pokémon Anime where Ash would be posing after catching Pokémon or winning a battle. The pose could maybe be an easter egg to the Super Smash Bros series, where each character also makes a victory pose when they win, giving them more realistic emotions.
The only source I used was this reddit thread.
"That's one of the cooler things about SM. Stop and go running and tripping is so minor, but it makes it that much more realistic." -Anonymous User