As some A(M)C 20-193 objectives aren’t indicated for DAL C systems, do I not need to perform related activities for DAL C?

It’s true that some A(M)C 20-193 objectives only apply at DAL A/B. However, the MCP_Software_1 objective requires determination of software worst-case execution time (WCET) even for DAL C systems. This will require some understanding of the interference channels on the platform and their impact on the software. You should speak to your certification authority to determine what level of rigor may be necessary for related activities. A white paper on the topic is on a shortlist to be added to a future version of MACH¹⁷⁸ Foundations.