There possibly isn’t one specifically for sega master system, since PicoDrive runs them. The Picodrive core is actually one that does a lot of Sega consoles!
It’s a bit messy, but you can actually just put your master system roms in the same directory as your megadrive/genesis/gamegear/sega cd/32x/pico etc roms.
There’s also a standalone PicoDrive app you can install for the gameshell.
Early on, there were some speed issues with Sega emulation, but I’m fairly sure most of them are now sorted out.