Jack Applin sends this in: “The caveman is pleased with the mammoth he drew. Is the mammoth jumping, afraid of a primitive mouse? Is it flying, with its Mercury-like ears, as an ancestor of the modern flying corgi (https://i.imgflip.com/66d5ta.jpg)?”
Comics I Don't Understand 
Kliban’s work is mostly absurdity for the sake of absurdity. Here, if there was a point, I would say it’s a contrast between the cleanly drawn mammoth and the rather hirsute caveman. Maybe.
Or it could just be a sketch he did. After Kliban’s death, a number of books were published that just seemed to gather everything that might have been on his desk–actual cartoons, drawings, and some rough sketches.
I happen to like his work, but it’s rarely in a “joke” context.
Ancient Gary Larson.
I think beckoningchasm has it. Kliban’s work is often hard to parse and almost as often has no real punchline. I would suspect here, though, that the contrast between the clean drawn mammoth (with the most famous variety of such creatures being “wooly”, of course) and the hairy caveman is the gag.
One of Kliban’s most famous cartoons has the folk-singer cat singing “Love to eat them mousies …”
There’s no punch line, but that never stopped it from selling on T-shirts and posters.
Love to eat them mousies
Mousies what I love to eat!
Bite they little heads off,
Nibble on they tiny feet!