I think Negan is just a dumb character. Lines like "Pee pee pants city" just don't work when spoken out loud. Past villains worked partly because they felt like they could exist in the real world. Negan, on the other hand, is like a living cartoon character. Like someone wondered what would happen if Andrew Dice Clay was dropped into a zombie apocalypse. It starts to wear thin. Plus, with Jeffrey Dean Morgan not being very big, you have to wonder why anyone would put up with him. At least if we was a big as he is in the comics, you could see why other, equally dangerous men would not want to mess with him.