Yes, it's grammatically correct, especially when zombies say it. In English, you often have a choice between whether to treat a noun as a count noun or a mass noun, and the choice conveys a subtle shade of meaning.
Count nouns and mass nouns
With a count noun, an indefinite article normally* introduces the singular form, and some goes with the plural, like this:
I bought a desk today.
I bought some desks today.
A mass noun normally* appears in the singular, without an article, and some goes with the singular like this:
You must drink water in order to live.
I drank some water half an hour ago.
Count nouns usually refer to things that naturally come in separate, discrete instances, which you can count: one desk, two desks, three desks, etc. Mass nouns usually refer to things that exist in continuous quantities, which are not susceptible to counting, like water, mass, air, space, continuous action, etc. If you think of count nouns as names for objects and mass nouns as names for liquids or materials, then you understand the central distinction. As you will see in a moment, though, the central distinction between count nouns and mass nouns can be reversed for deliberate effect.
The meaning of the choice
The fact that a word is normally a count noun doesn't mean you can't use it as a mass noun. If you treat a count noun grammatically as a mass noun, this means that you are thinking of it as something that exists in continuous quantity rather than in distinct, separate objects. By treating it as the mass noun, you convey this way of thinking about it to your listener.
For example, we normally think of brains as separate objects, so "brain" is normally a count noun:
George studies brains at the university, and he has a brain of his own, too.
But zombies (presumably) think of brains not as separate objects, but more like ground meat:
Notice that meat is a mass noun:
Vegetarians don't eat meat.
Would you like some meat on your sandwich?
It's not unusual in English for words for food to be used as both count nouns and mass nouns:
I ate a banana for breakfast. [count noun]
If you add some banana to that dough, it will taste even better. [mass noun]
We sold twelve cakes. [count noun]
We're serving cake and ice cream. [mass noun]
The choice depends on whether you are thinking of the food as discrete objects or as a material.
So, when the zombies say "Let's eat some brain!", they're really using the grammatical resources of English in the most exact and expressive way—expressing how they think about brain(s).
*I say "normally" here because, as usual with English, there are variations that convey other differences of meaning, so don't take the "normal" usage as a rule. For example, count nouns can be preceded by other determiners, like each, as in "each desk". You can use an article with a mass noun, as in "a water", but then you mean "a type of water". It's usually easier to master the main, "normal" uses before you learn the variations. This whole answer is about one of those variations: using a count noun as a mass noun.
The photo comes from warosu.