"Mamimimi!!" is a derivative based tag depicting Mami without a head, a dullahan. Though, this is NOT written by Gen Urobuchi, and it is likely to be unintentional as well.
Origin (Which Contains Spoilers)
Which goes without saying, the popularity of DRRR!! has infected the show, where fans immediately relate the loss of Mami's head with Celty Strulson, a dullahan of that series.
Puella Magi Madoka Magica Mami Tomoe
Celty Sturluson Dullahan
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