Faking rationality would be rather easy: Criticize everything which is not generally accepted and always find biases in people you disagree with (and since they are humans, you always find some). When "rationality" becomes a popular word, you can get many followers by doing this.

Here I assume that the popularity of the word "rationality" will come before there are millions of x-rationalists to provide feedback against wannabe rationalists. It would be enough if some political movement decided to use this word as their applause light.