There’s 2 parts

Part 1, no knock against Michael Cera, he’s a talented actor, but he kinda gets perpetually typecast into his role from Juno: awkward nerdy white guy wondering why life is happening to him

Part 2, Scott in the comic is, I can’t emphasize this enough, an asshole. He appears to be a Michael Cera character wondering why life is happening to him- except that he’s actually fully self-aware, and he’s just lying to himself and putting on the dopey act just so that he can get away with all his toxic bullshit. It’s only after he basically confronts himself on his own bullshit that he can move on with his life and relationships

Because the comic wasn’t done by the time the film started production, the movie basically misses out on most of the important parts of the ending and we were left with the cliff notes version.