They are degrees that people believe are useless and will lead to nowhere.
For example, some people say Media Studies are silly degrees or Film Studies, PE, Wine Studies, Happiness degrees (UEL do this!) cos they don't know where they could take you after uni. I'm doing International Business Studies with French and I remember some teachers looking at me in horror when I announced I was gonna do Business at uni. But as long as you know what you want to do with it (like I do) then why give a toss about what everyone else thinks? It's you doing the degree, not them.
In the 1800s, English was considered a Mickey Mouse degree (I can kinda see why) but it's one of the most popular courses in Cambridge. So what can ya do, eh?
It's mainly cos the degrees considered Mickey Mouse degrees aren't traditional but quite modern and vocational.