If I just say "dim" (not "dim down"), does it sound awkward?
Saying simply "dim" would not be awkward at all. Since the word "dim" means to make or become less bright I would say that actually "down" is kind of unnecessary in the original sentence from the clip. I think "dim half the lights" is more natural sounding anyway. Ex: Let's dim the lights and watch a movie. Ex: He's still afraid of the dark so I just dim the lights in his room.