The term originated on the LOGO Show, Rick & Steve.
Here's the official story of the Michigan Mop Job. Originally the line was "Plus I'll swallow because I don't have anything to lose." But Standards & Practices wouldn't approve it to air. In fact, they wouldn't allow ANY known euphemism for a sex act involving semen eating .
So I had to come up with a fake sex act that didn't mean anything. And that was actually tougher than you'd think. It seemed every word actually meant something: seattle sock job, blowfishing, cottage cheesing, california car wash... it was all taken. Finally I went with Chuck's moustache for the inspiration of a "mop job" and added Michigan to make it sound nastier, and it didn't show up in Google, so it was safe.
In short it's not actual act, but if it were, it would probably involve a moustache and eating ejaculate.