The cat in the cartoon might be afraid because it was separated from its owner or got lost. Sometimes, just a sudden shadow or an unexpected movement can startle a cat and make it afraid.
I don't think Siren Head is afraid of Cartoon Cat. They are both creepy creatures in different fictional universes with no established fear relationship.
It could be that the story or plot of the cartoon is too intense for the boy. Or perhaps there's a villain that looks really threatening.
He might be afraid because he's facing a big challenge he doesn't think he can handle. Like having to perform in front of a huge crowd when he's really shy.
The cartoon dog might be afraid because it's in a new and unfamiliar environment. Or perhaps there's a mysterious figure lurking around that's making it nervous.
It could be that the environment in the cartoon was unfamiliar and threatening to the dog. Or perhaps there was a mean character that scared it.
The guy in the cartoon might be afraid because he's being chased by a villain or has heard a creepy noise. Sometimes, it's just his overactive imagination creating fear for no real reason.
The little mouse could be afraid because it was lost in a dark place and didn't know how to find its way home. Or perhaps there was a loud noise that frightened it.
The reason could be that the cartoon has given the car a personality that values cleanliness highly and dirt is seen as a threat. Also, it might be a part of the story's plot to create tension or humor related to the car's fear of dirt.
Maybe he had a bad experience related to shots before. That could be why he's scared now.
Maybe she had some bad experiences in the past when trying to draw cartoons, like getting criticized a lot.