There is a lot of speculation, and even some informed-sounding
statements, on the Internet as to the origin of "Mickey Mouse" as a
derogative adjective. Some think that it relates to the poor quality
of Mickey Mouse watches. Another source traces it to American college
students of the late 1950s, who would describe an easy course as "a
Mickey Mouse course." Yet another goes back to World War II, where
soldiers apparently referred to absurd army routine as "mickey mouse".
"Expressions and Sayings: M"
Jeff Rice, Networked Writing Environment, University of Florida
"M-I-C-K-E-Y M-O-U-S-A", by Ellie Badesch (1999)
But the true origin seems to be in the jazz world. According to the
Online Etymology Dictionary (a source that I've found trustworthy in
"Mickey mouse (adj.) 'small and worthless' is from 1936, originally
used especially of mediocre dance-band music, a put-down based on the
type of tunes played as background in cartoon films ...."
Online Etymology Dictionary
For other references to this use, see:
Michael Borshuk, Department of English, University of Alberta
"Swing Era and Modern Colloquialisms"
Big Bands Database Plus
This use came less than a decade after Mickey Mouse was created!
Search terms used on Google, in various combinations:
"a mickey mouse"