It's something we've discussed on this board many times. There are different aspects of defense though so it depends on which one you want to talk about? Honestly, there are too many aspects for me to list so which one do you have in mind? Perimeter one on one? Low post one on one? Help? Getting around screens? Shot blocking? Rebounding?
Keep in mind, being a consistent great defender requires IQ, great fitness, at least some athleticism, determination, timing, a good sense of awareness as well as communication. It all might as well be a skill even though some of the aspects of defense doesn't require a skill level like shooting. So in a sense, yes, it is a skill.
With that said, you still need to put the effort in so if you have all those good qualities but don't try, you might as well just be Homer Simpson on defense. Kind of like Stephon Marbury back in the day.
“Whenever I’m about to do something, I think ‘Would an idiot do that?’ and if they would, I do NOT do that thing.”- Dwight Schrute