As well as a festival of Bavarian tradition, the Oktoberfest in Munich is a meeting place for people from all around the world -with very different cultures and divergent opinions with regard to fashion. Americans might not see the problem with wearing flipflops while, unless they're on the beach, the French wouldn't be caught dead without a proper pair of shoes..
However, when it comes to the traditional Bavarian costumes many visitors like to wear, it's the Munich hosts who make the rules! And once they've donned their lederhosen, shirts, and shoes, it's often difficult to tell where people are actually from. Alongside the traditional styles, however, younger Munich residents have taken a more open and casual approach to the outfit. In principle this is a good thing, and it's great to see locals and tourists playing with traditional dress codes to make a fashion statement or express their personality or own national traditions.
But not everything you'll see being worn at Oktoberfest can be put into the category of "individual taste". And some outfits meant to be light-hearted or humourous are actually complete faux-pas and downright offensive to Bavarians who love their traditional clothing. Even if nobody will actually confront tourists wearing cheap, ugly novelty "lederhosen" made of plastic or dirndls so short that they leave precious little to the imagination, they certainly won't be inclined to give them the same warm welcome they will extent to people who make a genuine effort to dress well.