Silver Screen wiseguys/JoePuzzles wrote a little piece on this myth.

"The term 'Mickey Mouse Mafia' was created by the Los Angeles Police Department as a way to mock the Los Angeles Crime Family post-Fratianno’s defection. While the term began to be used by newspapers at least as early as 1981, it really gained traction in October 1984, when the Associated Press published an article quoting Los Angeles Police chief Daryl Gates following the public announcement of the conclusion of ‘Operation Lightweight’. In it he referred to Peter Milano’s organisation (then Boss of the Los Angeles LCN Family) as the “Mickey Mouse Mafia” and felt the code-name of the investigation appropriate because, “organised crime is such lightweight in Southern California”. As such, it was an invention of law enforcement and the media.

Source: ‘Mickey Mouse Mafia’ bid busted by the Associated Press (published October 29, 1984) and ‘Super Snitch’ Did His Job by United Press International (published January 27, 1981)"

Here's a link to his fantastic website https://silverscreenwiseguys.wordpress.com