What does it mean to say that the sympathetic nervous system is organized for diffuse activity?
"Mobilize" in this context means to:


  to release (something stored in the organism) for bodily use


Merriam-Webster

"Diffuse" means "spread out": it doesn't say anything about whether the effect is "major" or "minor" and I don't think it's useful to think in those terms. Instead you should just appreciate that there is no specific target affected by sympathetic activation, rather it is a whole body response: almost every organ system is involved.

Sympathetic nervous system activation occurs when an organism needs to be ready to act immediately (such as in a 'flight or fight' response), which includes more metabolism in the muscles and increased sensory sensitivity. It also includes compensating by reducing metabolism in the gut: even though digestion is a source of energy, it is slow rather than immediate and costs a lot of energy in the short term through the processes of digestion.