silly or childish
puerile - silly
‘Puerile’ was originally used in relation to a child but has since been adapted to refer to immaturity or childish behaviour from an adult. It is a formal word that tends to be used derisively or disapprovingly.
I didn’t really enjoy the movie. I found the jokes to be a little too puerile for my taste.
His ideas were puerile nonsense. They were like something a child would come up with.
The concept was defined by puerility and arrogance. It was doomed from the start.