In a show it's the actor's job to portray the character he playing. It's the writer's job to improve and grow the character on the show as time goes on.
In Voyager the Kim character suffered the same fate as Denise Crosby who played (Tasha Yar) the cast was so big her character surfed and was not improved like the others. Crosby offend had fights with the writers about how lame her character was. This is why she didn't renew her contract and left the show.
In many of the episodes with Kim when they tried to grow his character. Like the episode when he fell in love with a alien women against Janeway's orders, but at the end of the episode it's like they hit the reset button and it never happen and Kim when back to be an upstanding officer.
Garrett Wang character also suffered from politics of Hollywood. Jeri Ryan was also one of the Co-producers and creative consultants on the show did not make it easy for Garrett Wang.
It's said that Jeri Ryan and Garrett Wang did not get along and the show.