time-awkwardness
Explanation
The Joke
The comic is titled "Best Use of a Time Machine: Recording History's Most Awkward Moments." It depicts the scene at Ford's Theatre on April 14, 1865, shortly after President Abraham Lincoln has been assassinated by John Wilkes Booth. While one might expect the time traveler to focus on the tragedy itself, the comic instead captures a theatergoer complaining: "Yes we all agree it's very sad about Mr. Lincoln, but I didn't pay full price for three-fourths of a show."
The joke is that even in one of the most shocking and tragic moments in American history, someone would still be petty enough to complain about not getting their money's worth for the interrupted play ("Our American Cousin"). A woman next to him covers her face in embarrassment at his callousness.
The Humor
The humor works by contrasting the enormity of a presidential assassination with the kind of trivial, self-centered complaint you might hear at any interrupted entertainment event. It is a commentary on the universal human capacity for pettiness even in the face of historic tragedy. The time machine framing adds an extra layer: of all the things you could use a time machine to witness, this person chose to capture humanity's most embarrassing moments of social gracelessness, suggesting that awkwardness is even more compelling than historical significance.
The votey panel simply says "Too soon?" -- a meta-joke about whether it is appropriate to make jokes about Lincoln's assassination. Given that it happened over 150 years ago, the phrase "too soon" (normally used when a joke about a tragedy is made before enough time has passed) becomes absurd, though the comic's premise of time travel makes the question of timing delightfully ambiguous.
References
President Abraham Lincoln was assassinated by actor John Wilkes Booth on April 14, 1865, while attending the comedy "Our American Cousin" at Ford's Theatre in Washington, D.C. Lincoln was shot during the third act of the five-act play, meaning the audience indeed did not see the complete performance. The "too soon" joke in the votey references the common social norm about when enough time has passed to joke about tragedies.