There's a dolphin zombie, but we never see the death of the dolphin, nor can you kill it in gameplay, as it leaves when its rider either jumps over a plant or dies himself.
Bungee Zombie dangles from a rope during the rooftop levels, which could be potentially triggering for those who are sensitive regarding hanging imagery
The mini game Zombiquariun takes place inside a fish tank, and it's filled with zombies. This same fish tank backdrop is used in the Zen Garden for aquatic plants.
Level 10 on fog is literally bright to dark flashing which is intentionally meant to be disorienting to the average player let alone to epileptics and photosensitive people. I had to make my roommate play that level for me. The sprites (character models) flash slightly whenever hit as well and in the final boss battle when the boss is deafeated the screen flickers
One zombie, the Jack-in-the-box Zombie, has a ripped straight jacket and is very shaky, implying he's an escaped mental asylum patient. The almanac calls him crazy.
Zombies can be run over by a lawnmower but it is not graphic. Plants can also be run over or flattened by various zombies/vehicles but it again is not graphic.