Maybe it's for a unique artistic style or to create a mysterious look.
Inside a dog's mouth in a cartoon, it could show sharp teeth for a fierce look, or a friendly smile with some saliva. Sometimes, there could be a piece of food the dog is about to swallow. It all varies based on the story the cartoon is telling.
It could be to make the girl's expression more ambiguous and leave it up to the viewer's imagination. Or perhaps it's a symbol for something the artist wanted to convey but didn't want to be too obvious.
Mouth expressions in cartoons can vary a lot. They can be exaggerated, simple, or very detailed depending on the style of the cartoon.
The cartoon dog might be frothing at the mouth because it's in a dangerous or threatening situation. Maybe it encountered another animal that scared it. Another possibility is that it ate something bad that's causing this reaction.
The expressions of cartoon mouth and eyes are usually depicted in a highly exaggerated and stylized way to convey various emotions.
Angry eyes and mouth in cartoons are often shown with narrowed, intense eyes and a wide-open, snarling mouth to convey the emotion.
Sometimes, the style of the cartoon influences how the mouth is shown. Also, it might be to make the character stand out or to communicate certain feelings more effectively to the audience.
Often, such a cartoon could portray the dentist with tools in hand, concentrating on the patient's teeth and oral health. Maybe there's a worried or calm expression on the patient's face too.
Maybe it's for a funny and creative twist. It could be to catch people's attention and make them laugh.
Maybe it's trying to hide a smile or keep a secret.