I believe it current supports 4-way split, which makes Forged maps impossible to implement. Now if they change that, then Forge maps are possible.
Judging by the lack of DLC in matchmaking in general, people don't buy DLC.
Good enough for MM? There are two maps in the Invasion playlist, and they are both pretty poor (Spire is an abomination). I'm not typically into a huge injection of forge maps into MM, but in the case of the Invasion playlist, it needs it desperately, and I've played some pretty good forged Invasion maps.