It turns out that Nyarko-san isn't just the moe personification of Lovecraftian horror Nyarlathotep, she's also apparently a fan of live-action Japanese special effects shows, known as Tokusatsu. The long-running Kamen Rider series in particular seems to be the Rosetta Stone for how she/it is configured on the covers of the original Manta Aisora and Koin novels.
On the second through fourth covers, she replicates the poses of the first three seasons of the Kamen Rider franchise, which started in 1971, and became Kamen Rider V3 with the 1974 sequel. On volume five, she breaks the combo, and the trend of Showa Era (pre-1990) shows with 2001's Kamen Rider Agito. She jumps back to fourth show, 1974's Kamen Rider Amazon, for volume 6, then its the tenth, 1982's Kamen Rider ZX, for volume eight.