"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)
----
Related work
Related illustrations
Related tags
Puella Magi Madoka Magica Mami Tomoe
Celty Sturluson Dullahan