This was to celebrate Mami's 250th thread on 2ch, which is a very high number for a character.

Everyone loves Mami!

• Datalogs:
• http://read2ch.com/r/anime2/1333162613/#t57
• http://log.shipweb.jp/?mode=datview&board_name=anime2&thread_key=1333162613&thread_id=398551#res57

2ch users call this anonymous artist ぽんぽこマミさんの人. It is Nori Senbei's fan-nickname. See also this pixpedia article and Nori Senbei's twitpic.

EDIT: According to 2ch's datalog, this image is a collaboration between Nori Senbei and another artist. Added collaboration and collaboration_request.