As an architect who's studied architectural history and theory in Rome and been to many baroque and rococo buildings, I would disagree. I find these types of buildings, if seen in photos or from a certain point of view (St. Peter's looking from above in the dome) they are very visibly huge. However, since they are built around the proportions of the human body specifically, I've often found them to feel much smaller and more intimate when you're actually in them.
To me, though, I'd think of Melkor's style (in scale as well as intention) being more like Fascist Italy's monumental structures: referring to the architectural forms of the Renaissance, Classical Rome/Greece, but largely breaking their rules of proportion in order to create bombastic and overblown forms.
In the sense of ornamentation, Baroque and Rococo might work, I don't know how Melkor might feel about such things (personally, I'd think he'd be a minimalist). In terms of theory, however, both would work fairly well as the form and decoration are meant to create sense of awe...of the sublime and a sense of something greater and outside oneself. I can see this easily be perverted by Melkor for his use and self aggrandizement.