In the real world, -氏 is definitely a strange name suffix among friends. But Sudō is a teenager girl in a literature club in an anime. Female teenagers generally like inventing a funny way of speaking, and being in a literature club means she is familiar with -氏 used in "serious" literature works. On top of that, this anime itself is mainly targeted at anime otaku, and a fictional character in such a work often speaks in a peculiar way, anyway, as you probably know.
By the way, you said "Sudō is everything but an otaku", but at least in Japanese, オタク can refer to nerdy people in general. An avid literature fan, a railroad fan and such perfectly qualify as an オタク in the broad sense, and a literature club is often considered an オタク系 club activity. Having a bit of the stereotypical otaku trait is not strange as a member of a literature club.