When you watch a zombie film, are you looking to be thrilled, or just to have some fun? Do you want to be scared for the sake of the characters, or do you just want to see some over-the-top zombie effects? There is no law that says that a zombie film must be a certain way. Different directors have different visions, and two equally watchable films can have entirely different atmospheres.
Lucio Fulci, the Italian director known for his dark and gory films, tries to make the audience feel disturbed. The zombies are disgusting, the protagonists are human and vulnerable, and the general atmosphere is very unpleasant. His movies are not completely devoid of humor, but the humor is only a trimming on the main horrific theme.
On the other hand, Sam Raimi of Evil Dead fame made his trilogy sillier and sillier until in the last one, Army of Darkness, even the cartoonish zombies are playing straight-man. The hero is a caricature, but taken to such an extreme that he is a satire of a caricature. The elements of the zombie film are there, but they are portrayed in a way intended to make the audience laugh.
Sometimes a film is made with the best intentions of being scary, but poor production makes it humorous instead, and camp is born. Ed Wood's Plan 9 From Outer Space comes to mind, where paper plates on visible wires were used to simulate flying saucers, and the acting was hammier than a third grade play.
The question is not "Which is the better zombie movie?" They are all equally valid methods (except for camp). The question is "Why did these directors choose to make these movies the way that they did?"
Raimi has said that his goal is to get a reaction out of his audience, and he realized that horror movies were an excellent tool for manipulating emotions. One of Raimi's big influences, however, was the Three Stooges, so in his first big horror film, Evil Dead, his horror had a slapstick tinge. In the sequels, he wanted to make the slapstick and humor more prevalent, and it became the dominant theme.
Fulci, on the other hand, felt that if you're going to make a film about the walking dead, it better be dark and disturbing. He tried to make his audience react by making his gore scenes as bloody and realistic as possible, and his storylines are filled with desperate characters.
So, it seems that the main criteria for being a good zombie film is that it must provoke a reaction, whether it be horror or humor. If the audience is drawn in, and their emotions are affected, the director has achieved the desired goal.