Suppose $X$ and $Y$ are two smooth surfaces (over the complex numbers), and $f: X \to Y$ is a finite flat morphism of degree two. Is it necessarily true that the locus where $f$ is not a smooth morphism (i.e. the ramification locus) always one dimensional (if not empty)?

Yes. Otherwise there would be an isolated ramification point in $Y$. A link of that point is $S^3$, which does not have a nontrivial double cover. So upstairs, $X$ looks near that point like two ${\mathbb C}^2$s glued at a point, which isn't smooth. 

