This has always bothered me. I mean, I guess it's irrelevant, seeing as how neither one of the characters given the morphing power (Animorphs, Andalites, Auxliaries, etc) have braces. But let's think hypotheticals for a second.
Imagine a world where one of the Animorphs (Marco, most likely, since Tobias' uncle is too cheap, Rachel is aparently naturally beautiful and perfect, Cassie would probably criticize the orthodontist) has braces. How would they morph? Would they braces morph with the person, like the skin-tight clothing? Is there an orthodontic practice set up somewhere in Z-space? And what about the permanent retainer?