It's worth pointing out that if you want to run MAME, specifically, RetroPie is a bad choice, as is using the Raspberry Pi in general. MAME's architecture does not lend well to it being converted into a core. Several features get weakened, disabled, or broken, and they generally don't function the way they're supposed to, especially with respect to controller input. The version of MAME available for RetroPie is consi…
It all depends on the era of game you want to play though. At least for me, I can't imagine a retro game I would want to play that a Pi couldn't easily handle, given that my idea of a retro game is Pac-Man, Galaga, Donkey Kong, and the like. Granted, if your idea of a retro game is a PS2 game from 2001, your mileage might vary.
It's also that the versions of MAMEs shipped are simply buggy, and have some features disabled. If you go digging around in the MAME menus, lots of things are not going to work properly.