It's an aftermarket add on. My 13 X3 35i M Sport doesn't have it.
But my M5 does.
Just a personal opinion, do as you like, but I'd never slap an M on the outside of a car that was not one. Even though many have spent enough on M products to justify doing it; it still just seems fake to me. But I don't begrudge someone that wants it.
I just wish my X3 M sport steering wheel had the red and blue stitching to go with the M emblem on it.
1979 Porsche 911SC
1996 Porsche 993 C4S
2003 BMW M5
2013 BMW X3 35i