They are subtly different. "As much" (in this sense) would NORMALLY be in comparison to something else. For example, "It is important to write interesting characters and dialog. What you write about doesn't matter AS MUCH".
"That much" (again in this context) is more absolute. It means that the subject that you write about is not particularly important. It isn't necessarily comparing the subject to some other specific aspect of the writing the way "as much" would.
However "as much" COULD be, and often is, used as a substitute for "that much". Both would be understood.