XKCD Isn't Funny - #1556 - The Sky
So, this comic isn't funny; but, to be fair, it's not trying to be, that's not the point of this particular piece. The point is for people to read it, think "Oh, wow, that's not a stick figure comic, Randy can actually do some pretty art stuff when he tries." and also perhaps "wow, that's deep that the sky is just the other half of the earth, such a fitting image for such a deep thought."
I'm actually not sure if I got that right, since the line "It's one of my favorite halves." is so vague as to what the other half might be. The title text indicates it's either the earth or the ocean, and either way I'm mildly surprised. I'd have expected Randy to be more scientific about the sky vs. outer space thing after reading #1394.
Taking the art on it's own, it's not a bad picture, it'd probably make a nice desktop image at least. It's got some nice contrast going on. The perspective is maybe a little wonky and the smaller clouds look like he was just filling in what he saw as empty space, but overall the second panel is good job at making pretty thing, Randy. I like the top left corner the most, myself.
Unfortunately, the comic's biggest flaw is right above that corner. It really isn't nice, compositionally speaking, to have a panel jutting out over the top of the large central panel. Maybe to fill up that ugly blank space, there could be a few extra panels showing them walking to the sky-viewing place, and a beat panel between Hair's two lines. It'd be a nice square, and the row of panels on top would even balance out the black ground on the bottom.