While I support using Pokemon you love, Pachirisu was a highly tactical choice. Se Jun Park said in a post-game interview that he needed a Follow Me user and was originally going to use Amoongus (for Rage Powder), but realized that other competitors would be prepared to counter that. Pachirisu caught everyone unprepared and was only threatened by Garchomps. Se Jun was prepared to take down Garchomps with Intimidate + Shadow Tag, a Garchomp of his own, and a choice scarfed Gardevoir. Strong Pokemon and weak Pokemon is a naive perception of people. Skilled trainers will look for unexpected team combos and outwit the opponent. Bonus points for using your favorites, but I don't know if that had anything to do with Se Jun's choice.