I'm red-green color blind so I won't weigh in on the red plastics. All red looks subdued to me. If you have RGB lighting, maybe giving those GI lamps a slightly red hue may help it pop.
What if you put another Cup Head and Mug Man logo on each pop bumper cap? Kinda make it look like they're beating up on the devil. But agreed, match the font with the rest of your playfield for consistency if nothing else.
It may be the angle of the camera and shape of the plastics, but the dice man art above the left drop seems a bit smaller than the devil one above the right drops. Have you thought about swapping the art above the drops with the sling art? Further associate Cup Head and Mug Man with their target areas, and bad guys are the slings trying to end your ball.
Also just curious about the bonus ladder, do you progress in the same order of bosses every time through the game?
Kudos on all your progress. Seems like the finish line is within sight.