Thermal effects can impact any platform, not just multicore platforms, and DO-254 currently has no additional considerations for multicore systems. While a revision to DO-254 is upcoming in which some multicore concerns may be addressed, it may be sensible to assume that on-target testing for A(M)C 20-193 should take environmental testing in mind.
A(M)C 20-193 objectives state that worst-case scenarios are implicit for WCET analysis, which implies ensuring the objectives are met in the worst-case conditions the system will be subjected to. For example, thermal throttling of one core may cause operating frequency changes in other cores.
Dynamic voltage and frequency scaling in general is out of scope of A(M)C 20-193.