We'll just let Seth Grahame-Smith (the real person, not the narrator) explain this: "The more absurd your premise, the more serious your execution has to be" (source).
To be fair, the book has some humor, like when Poe thinks Lincoln is a vampire because he's shielding his eyes from the sun, when in fact Lincoln is only sensitive to light because he's hung-over (4.47, 4.55). That might give rise to a brief chuckle, but don't base your entire stand-up routine on that idea.
See, brief chuckles are just about the only chuckles to be had in this book. Overall, the tone is super serious, with footnotes adding facts that we might want to know to give it a little historical flair. For example, when Abe first dedicates himself to killing vampires, it's a pretty dramatic moment. He writes that he has one purpose in life: "That purpose is to kill* as many vampires as I can" (2.93). What an awesome line. But what's with the asterisk in the middle?
Oh, that asterisk marks a footnote that tells us something totally undramatic:
It's interesting to note the repeated use of the words "kill" and "killing" in these early entries. Abe would later use the more accurate verbs "destroy" and "slaughter."
Maybe that's interesting (not really) or useful information, but it's not very dramatic. Why slap a boring footnote in the middle of a pivotal moment for Abe? Because Seth Grahame-Smith is super serious about his superhero.
This book is all about proving to us that our version of Honest Abe is a lie, and that this version—the vampire hunting one—is the real Abraham Lincoln. Sometimes, to gain a bit of the readers' trust, you really just need to drop some knowledge on them, in the form of a footnote bomb. Well, played, Seth. Well played.