Well, originally Misty didn't want to capture Psyduck because honestly would you want a Pokémon with a serious chronic illness (Psyduck's constant headache)? And I really can't blame her for being annoyed with it popping out when she called on a different Pokémon, and it not being a really powerful battler until it's headache was really severe. And as for her hitting Psyduck, she had to make it's headache really bad for it be able to use it's powerful Psychic attacks, so she attacked it in various ways like punching and hitting it, or even slamming a bike over it's head.