Michael Cera is the best casting for this character because we can immediately see who he really is right from the start. There’s no illusion for the viewer that he’s actually cool. He pretends to be cool, he aspires to be, but he sucks and he knows it - though he doesn’t realize how apparent that also is to everyone around him.
In a comic you can convey that quickly through text and other means, but if you want to let the audience in on how tragically lame he truly is right up front, there’s nobody better at that than Michael Cera.