I doubt any are available in the App Store. NSFs are, to my understanding, stripped-down ROMs that are just the music engine of the game from which they are ripped. To execute them, then, requires a NES emulator, albeit one not needing PPU functions. And to run your NSFs, it is having to run code provided from outside itself.
(the gstreamer NSF vulnerability would be an example of why they actually have modest grounds to be concerned, particularly with walled-garden thinking.)
2.5.2 Apps should be self-contained in their bundles, and may not read or write data outside the designated container area, nor may they download, install, or execute code, including other iOS, watchOS, macOS, or tvOS apps.
(slightly changed from last year:)
2.7 Apps that download code in any way or form will be rejected
2.8 Apps that install or launch other executable code will be rejected
That said, they're only human, they could have missed this or not understood the implications of the NSF format and passed one through, so I wish you luck on your search (and am also interested in any results!)