It's true that a stylized fish symbol is widely used today as a symbol for Christ or Christianity, but I'm not sure that I see Christian symbolism in Vardaman's statement in As I Lay Dying. Rather, his statement seems to me to reflect his way of making sense of his mother's death.
Before making his famous statement that his mother is a fish, Vardaman is reflecting on his experiences with catching and cutting up a fish. The fish is alive when it is caught, but is no longer alive after it have been cut up. Vardaman seems to me to be using his limited experience with life and death (in the form of the fish) to make sense of the loss of his mother.
Symbols are multivalent; they can mean more than one thing. There are other meanings of the mother/fish in the story, and there very well may be Christian symbolism here, too. Perhaps someone else will be able to point it out.
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