Q: Is it okay to use a frequency modulated (FM) source as my clock input to the system clock of the DDS
A: No.
Typically, the DDS system clock is the timing source for much of the internal workings of the DDS. A system clock with FM can introduce excessive jitter. Too much jitter will cause timing problems within the DDS device and significantly degrade performance. This is applicable to both PLL disabled and PLL enabled. Thus, the clock source must be a stable single-tone frequency source.