When was the Red Baron? is an interesting bit of research by Brett Holman. The basic point is this:
Nevertheless, it seems that up until 1960 or so, it was quite rare to call Richthofen the Red Baron. Conversely, from 1970 or so, it was almost impossible not to: I don’t think I’ve found a single book about him from the last half-century which does not also have ‘Red Baron’ somewhere in the title.
Spoiler: Snoopy is to blame.