"The Room" (2003, Tommy Wiseau) is actually a good movie. A great one, in fact.

In “The Disaster Artist”, is James Franco’s character supposed to be mentally handicapped? Or is that just how he appears in the trailer?

That’s how he appears in real life.

Franco is doing a pretty faithful imitation of Wiseau. Wiseau is an extremely odd man and precisely why he acts that way is not really well known.

I’ve heard theories that Tommy Wiseau has brain damage, it would explain some of his behavior. He apparently does have memory problems, like for one infamous scene on the roof where he absolutely couldn’t remember his short line.

Part of this might be a language thing. Wiseau’s background is obscure (I’ve always thought he was East European - he’s claimed to be Cajun, among other things), but he reminds me of French Canadian Georges St Pierre in his speech.