dumber than a mud fence...... to build a fence out of mud would be stupid, because the mud is easily broken, and will easily fall apart with time or rain. So he is saying someone is dumber than doing something as stupid as trying to make a fence out of mud.

example: Tikolx333 is dumber than a mud fence, because he likes reposting, and than enjoys getting flamed. it makes no sense to make a mud fence, and it makes no sense for him to continue posting ancient threads over and over again.