Children and adults
In this story, Marco and his father disagree about the importance of imagination. While Marco prefers to look at the world through the lens of fantasy, Marco's father discourages this. As such, the interaction between Marco and his father can be seen as a metaphor for the disagreements between children and parents.
Imagination
Marco creates a number of fascinating creatures and situations, using his imagination. In this sense, we might see Marco as a metaphor for children, who see the world with a sense of wonder.
Mulberry Street
In this story, Marco takes issue with the reality of Mulberry Street, seeing it as boring. He instead decides to see this setting in his own unique way, inspired by his imagination. As such, we might see Mulberry Street as being a metaphor for the dull realities of everyday life, which might benefit from a bit of imagination.
Eyesight metaphor
Marco's father tells his son that his eyesight is "much too keen," a metaphor for the fact that Marco sees things that aren't really there due to his strong imagination.
Red as a beet
Seuss uses a simile to emphasize Marco's embarrassment when he is caught telling lies: "Nothing' I said, growing red as a beet."
