Background music has one job
The best background music is the track nobody notices, because it supports the picture and the voice without competing with either. In practice that means a steady rhythm, a soft or warm timbre, a melody that does not take the lead and no sudden changes that make the viewer look away from the screen.
That is the opposite of how most songs are written, so prompts need to say it. Ask for instrumental, a consistent energy level, no solos and no dramatic drops. If you give the generator a mood plus the instruction to sit under a voiceover, the arrangement tends to stay simple and the mid-range stays clear.