The comic of course is a play on the Where’s Waldo? children’s books. Then, there is the current phobia about anything even vaguely Muslim, Islamic or Arab. “The sky is falling! Terrorists! Mosques! AAGGGH!” You could also consider it a snark on the Muslim ban against depicting the image of Muhammad anywhere. Or the unsale-ability of the book title.
But, there’s another angle. You can’t find Muhammad (as in a common name for a Muslim guy) in the picture, because he probably looks just like anyone else. Just a regular guy, and not some crazy extremist ready to force sharia law on US citizens.
Of course, if you’re paranoid or watched too many spy movies, maybe you’d think Muhammad is disguised as the ice cream guy, who’s got a bomb in his cart. If you think that way, you need to get out more. Seriously.