I suspect that crab apple is just fine for smoking with. However, I've never tried it or talked with anyone that had, at least not that I can remember.
I did a quick Google search and the most common response was:
"Crabapple is a good flavored wood very similar to apple but with its own unique fruity flavor."
So I say give it a try. I would just add a couple small chunks the first couple of time and mix it with something else. Let us know how it turns out.
I've got a flowering crab in my front yard that I've used dead limbs from for smoking and it is very good. Similar to apple. The tree is starting to die and I will miss seeing those beautiful blooms in the Spring but it'll make lots of smoke wood.