It looks like you can forget about Game Room ever having mature games for you.

In a move to make things easier and cheaper for publishers, Microsoft has decided to get the entire application rated, rather than getting each individual game rated. Of course that's a smart move in some way as you won't get leaks on f.i. the ESRB or PEGI site, but that also means you can only put up games with the same rating. And that rating currently is E.
Seeing as most old games from the 1990s are rated E or E10+ this shouldn't immediately pose a problem, but what about newer titles? Or games like Mortal Kombat? Unless Microsoft changes their vision or the ESRB/PEGI/Whatever does a periodical check, it looks like such games will never make it.

In other news: