A haiku uses
five-seven-five metre, not
seven-nine-seven

Normally, I prefer translating "nyan" to "meow" or similar myself, but this time, it's specifically a pun on Seiga's title. (See her Touhou wiki article for details.)

Eromame's Seiga and Yoshika are both so damn adorable

CoCoMoo said:
Eromame's Seiga and Yoshika are both so damn adorable

Actually everyone drawn by Eromame is goddamn cute

Moonspeaker said:
Normally, I prefer translating "nyan" to "meow" or similar myself, but this time, it's specifically a pun on Seiga's title. (See her Touhou wiki article for details.)

That and nyan-nyan is old japanese slang for sex.