until recently, *DOOFUS* was not a word in the english language, while *IDIOT* can trace its roots pretty far back.

so doofus is:
An incompetent, foolish, or stupid person.

while idiot means something similar, depending upon context:
1. A foolish or stupid person.
2. A person of profound mental retardation having a mental age below three years and generally being unable to learn connected speech or guard against common dangers. The term belongs to a classification system no longer in use and is now considered offensive.

as a localism / colloquially? doofus and idiot are used interchangeably, not just for fat stupid white people, but also for fat stupid chinks, japs, kykes, and wetbacks.