You know now that I think about it, I wonder how many M-rated games were released on the Game Boy/DS systems. I know some of the obvious ones would be id Software ports (like the Wolfenstein 3D seen in the picture) and the GTA games (both the GBA one from the mid 00s and Chinatown Wars from later), but other than those I can't imagine the list is a very large one, especially considering most of the people who would have been playing on those systems at the time would have been kids and teens.

Edit: after a quick google search, it looks like there were a grand total of 9 M-rated GBA games released, most of them being not very good ports of more popular console/PC games (though an exception apparetly goes to Max Payne, which managed to successfully convert the original third person shooter into an isometric game). Similarly, only 11 M-rated games were released on the DS, the best and most well known of them being Chinatown Wars.