Did my surgery cause my midriff fat that won't budge? (Photo)

Dave.  Hard to see exactly what you were describing in your photograph, so physical exam would be helpful. Sometimes an abdominal scar like the one you have from your appendectomy can become contracted and cause issues that you are describing. You are obviously fit, however micro liposuction around the scar or excision of the scar with scar revision are options. Be certain to consult with a board-certified plastic surgeon in your region. Good luck!

I do not think the lack of definition in this area is related to your previous surgery. We all tend to accumulate stubborn fat in certain places. It appears you store some excess fat in the infraumbilical area, as do many men. Targeted liposuction of this area would probably make a difference for you.
It also appears you may have a slight amount of scar tethering that is accentuating the transition from above the scar to below the scar, like a shelf. This may be improved by a scar revision or by freeing up the scar from underneath while doing the liposuction.
Emile N. Brown, MD
Johns Hopkins & Harvard Trained Plastic Surgeon