The tag is created after the broadcast of episode 3 where the main enemy of this episode, Charlotte is thought by fans to be shaped like a Ehoumaki sushi.
That, and since to them, the fans also realized that Puella Magis fighting witches are the same as survival to the fittest, hence the tag.
Probably made out of denial, the tag is mostly illustrated as Mami eating Charlotte (which is made out of the shape of Ehoumaki sushi). Perhaps they just can't accept the fact that she is gone...
Also, regardless of the appearance of Mami, there exists food pictures where someone made a food with the shape of Charlotte as well.
That poor witch...