A speaker is deemed to be 'Mirrorgirl' when they miss a subtle joke and then either go on to make a less subtle version of the joke themselves (imaginging themselves to be creating original material), or otherwise set out to explain the joke without realising it is meant to be funny. The correct response to such behaviour is 'Thanks Mirrorgirl'.
When used online, commonly abbreviated to 'TMG'.
Mr A: It seems to me that it's only once people have got a regular job and a stable home life that they start thinking about Maslow.
Ms B: Isn't that the point of Maslow? You can't start properly building family/esteem/etc till all the basic needs are taken care of.
Mr A: Thanks Mirrorgirl.