Satan
Explanation
The Joke
A person is standing in front of a large dinosaur skull fossil and complaining: "All these fossils! Put here to fool us into believing evolution and denying the LORD!" This is a reference to the real-world Young Earth Creationist argument that fossils were placed by God (or the Devil) to test believers' faith.
However, the person then pauses and reconsiders: "Wait, wait a sec. The Devil made a design so intricate, so perfect, so consistent with what you'd expect to see via a 4 billion year long chain of evolution? And God made... what, cows?" The caption at the bottom reads: "Inadvertently, Billy is convinced to worship Satan."
The Humor
The joke takes the creationist argument that Satan planted fossils to deceive humanity and follows it to its logical conclusion. If Satan created the fossil record -- an incredibly detailed, internally consistent, and scientifically coherent body of evidence spanning billions of years -- then Satan is actually a far more impressive designer than God, whose creations (like cows) are comparatively mundane. By the creationist's own logic, the Devil's work is so magnificent that it inadvertently makes a stronger case for worshipping Satan than God. The humor lies in the ironic self-defeat of the original argument: the more one insists that fossils are an elaborate deception, the more impressive the deceiver becomes.
References
The comic references the Young Earth Creationist claim, sometimes called "Omphalos hypothesis" or "Last Thursdayism" in its more extreme forms, that fossils and other geological evidence of deep time were placed deliberately to test faith. This argument has been used by various creationist groups to dismiss paleontological and geological evidence for evolution and an old Earth.