This comes from anything considered trivial or petty. The term comes from WW II and Walt Disney's famous cartoon character. At the time, the US Navy established a number of 'boot camps', which were officially termed, 'Military Indoctrination Centers', or MIC's. The discipline, regulations, and restrictions placed upon the new recruits were not very welcome, and they quickly combined the official term with the cartoon to denote anything ridiculous, non-sensical, or unimportant, but which had to be followed or obeyed.

Since a good deal of your time in boot camp is covering things which one might consider trivial or petty and new recruits are called MIC’s, the term grew out of that. From my own experience I can 100% say items covered in boot camp may seem Mickey Mouse, but they are important.

I am sure you have heard a person at work or even out in public say something about “dealing with all this Mickey Mouse stuff.” If you ever do hear it see if the person saying this is an older person, odds are they are.