I think they do. With the amount of time and preparation that go into them as well as the sheer imagination, I say customizing action figures is an art.
Here are some of mine.
I have a custom movie Silver Surfer.
All I did was take the head off of a Marvel Legends Xavier and paint it silver and put it on the body. Then I used the body from that same figure to make a super poseable movie Xavier:
I had to do tons of dremmeling to get his head to fit on the peg though.
And I also made a custom movie Doc Ock. I took some bendy tentacles from another Doc Ock figure and put them on the Tentacle Attack Doc Ock (because the Tentacle Attack Doc Ock has a better sculpt).
Again I do consider this an art. Some may not agree, but I think art is anything that you put a lot of time, detail, and passion into and create something worthwhile.