There is a concept in games (video and otherwise) called the 'magic circle' which delineates the boundaries of the rules. Within the magic circle, disbelief is suspended and the game exists because the players agree on the rules. "The Floor is Lava" game works because everyone playing agrees that the floor is lava and jumps across the room on couches. Everyone hates the kid who walks on the floor with magic shoes or whatever because he has stepped outside the boundaries of that circle. Likewise, if everyone was playing tag and some kid came in and said "Well Jimmy can't walk and the ground because it's lava to him, and Sally has to hop on one foot because she is a flamingo, and I have super powers and can tag from a distance" it wouldn't work because there is no consistency.
The same concept applies to works of fiction. We all suspend our disbelief for the zombies as long as there are consistent rules that are followed. A lot of people hated the zombie that bit Hershel because he didn't behave like every other zombie that has reanimated as a reaction to people walking by. Similarly, there is no reason that the characters should have gained super head shot mode as it isn't consistent with the world that is set up (namely, the real world + zombies). If this was a TV show based on the Marvel Zombie Universe, no one would complain if Carl became a marksman because you could say "oh that's his superpower" and it would fit within the magic circle.
Suspension of disbelief is not an excuse for crappy writing. The reason people get bothered is because this isn't some campy B quality show on SyFy, it's a high quality, high production value show on a network with other top shows.
