Titles are tricky business. They can allude to other works, just like Bradbury's Something Wicked This Way Comes, which comes from Macbeth. Then there are titles that point to the important character of the work. Shel Silverstein's The Giving Tree goes this route because the story's main character is, you guessed it, a tree that gives. Still other titles tell the reader to focus on the setting of the story. For example, Maurice Sendak's Where the Wild Things Are lets you know that the story takes place, um, where the wild things are.
So what about Marvin K. Mooney Will You Please Go Now!? First glance might suggest that it follows The Giving Tree by telling the reader to focus on the character of Marvin K. Mooney. And that's certainly part of it. But if that were all, why wouldn't it just be called Marvin K. Mooney? Why add all that other stuff?
Our guess is that Seuss is trying to keep the focus away from Marvin K. Mooney. Yes, really. Maybe Seuss is trying to say that the act of asking Marvin K. Mooney to go home is the true focus of the poem. After all, Marvin doesn't really do much. Sure, he's present the whole time, especially in the illustrations. But it's the Narrator's imagination performing all the actions, and his goal is to ask Marvin K. to leave. Inventing all the ways Marvin K. can go home takes up the bulk of the poem.
It's the act of asking Marvin K. to leave that Seuss wants the reader to focus on, and that's why this request belongs in the title spot.