If you mean it in the loosest definition, as in a portable device that can do computation, then yes, but obviously that is so vague as to be useless.
With a more strict definition, like every other question of this sort, it depends on usage; for some, it is more computer than they need. For others, it is inadequate. If you consume content, it's probably fine; if you create content, it's probably not. For me, I have this wacky thing I like to do called "copy/paste"
so for me, the iPhone falls far short.
My HP2133; THAT'S a portable computer.