why are psychic types that look the most brainless of their kind the most overpowered
funnily enough there is actually a scientific answer to this!
psychic type proficiency in pokemon tends to present in one of two ways. the language around it isn't standardized because it's a difficult topic to qualify objectively, but a well-regarded paper by murray et al. ("the categorization of psychic-type moves by energy output and cognitive activity," published 2001) uses the terms active and latent proficiency, so that's what i'll use.
many psychic pokemon have active proficiency. their psychic capabilities, while an inherent aspect of their biology, are something they hone through repeated practice. this brings to mind pokemon like kadabra, which wield their psychic abilities with great skill and may even manipulate them with the help of objects (such as kadabra's spoon). these pokemon primarily use their powers actively and tend to develop good control over them.
other psychic pokemon have latent proficiency, which is described as psychic energy that is extremely powerful but largely lies dormant. espurr, slowpoke, and psyduck (despite not being psychic type) have this sort of proficiency. latent proficiency sometimes requires the pokemon to hold itself back, and sometimes it simply comes and goes, and when it's finally released it can be explosive. because of the sheer intensity of this energy, pokemon that experience it more frequently develop a "dopey" expression, either because of the recovery period from releasing that energy or because holding it back takes so much focus that they don't have much processing power for anything else.
active proficiency is much more common, but there are certain moves that rely on latent proficiency, such as future sight. most psychic pokemon are capable of latent proficiency to a certain degree, but the vast majority of them actively manipulate their psychic powers most of the time.