You ever hear an announcer describe a pay-per-view wrestling match in a commercial? They use terms like, "anything goes," "no rules," and "exciting." Well, if you take out the overcompensating, testosterone-driven tone of the whole situation and add in a voiceover that's a little more cheery and playful, you've basically got Seuss's attitude toward writing.
If I Ran the Zoo is all about imagination. And what are the rules and limits on imagination? Nothing that we can think of. It's anything goes, and Seuss's tone needs to be able to match these aspects of imagination. Everything should fun and exciting, and there should no rules, so to speak. Mission accomplished, Seuss.
The tone helps put the reader in McGrew's mindset. If not for the tone, we might try to stop to consider the logic of what McGrew is trying to do and how he's trying to do it. And that's a problem because there isn't any logic to it. It's just good, smart, fun. For example:
I'll bag a big bug
Who is very surprising,
A feller who has
A propeller for rising
And zooming around
Making cross-country hops,
From Texas to Boston
With only two stops. (24.1-8)
Why question something as great as that?