I must confess I thought Irma/Irving was "pretending" as well, but not quite in the way Sarah meant it. S/he is undoubtedly trying on identities in the way that a teen will go from a preppy to a metal/goth look, to something else entirely. I do understand Sarah's confusion--this would appear to her for all the world like one of Irma/Irving's games, such as pretending to be a superhero. 

On the other hand, I never thought Sarah was pretending. She's been quite resolute in her identity during this strip's entire run.