What's lazy about hiring a great actor? They're going to have to explain why he looks like Iron Man, so to me that's the opposite of lazy. They're creating more work for themselves by hiring RDJ for a role that's not Iron Man.
Except Doom's face never has to be - nor should be - seen. If we go with the comic version of Doctor Doom where he was disfigured in an explosion, that can simply be referenced and the mask never comes off.
I don't have any problem with that because that's how it was in the comics. I don't remember ever seeing his face and I don't believe they ever showed it. Even in the Fantastic Four comic from 1964 where they gave his origin story, his face was never shown. In the couple of panels featuring him before the explosion, we only saw the silhouette of his face in shadow.