Crab apples are meant to hold on tightly to their fruit, though there are some exceptions, because the fruit are part of the display. This can make them annoying for cidermaking, they don't pull of easily. I have a lot of crabapple trees but I only use the larger fruited ones for cider because picking the small fruit ones is just too much effort. Still, if that is all you have to add a bit of tannin, it might be worth it. It is also worth giving the cider some time to age because the tannins will soften a bit with time.