The Jargon File (version 4.4.7, 29 Dec 2003):
dumbed down
 adj.
    Simplified, with a strong connotation of oversimplified. Often, a 
    marketroid will insist that the interfaces and documentation of software
    be dumbed down after the designer has burned untold gallons of midnight oil
    making it smart. This creates friction. See user-friendly.
The Free On-line Dictionary of Computing (30 December 2018):
dumbed down
    Simplified, with a strong connotation of
   *over*simplified.  Often, a marketroid will insist that the
   interfaces and documentation of software be dumbed down after
   the designer has burned untold gallons of midnight oil making
   it smart.  This creates friction.
   See user-friendly.
   (1995-04-14)