No more than the Tumbler driving on rooftops, or the Bat-Pod coming out of it.
Well, the roof scene is plausible, depending on the particular characteristics of that enviroment. The Bat-Pod coming out of the tumbler simply represents an advanced state of technology. Seems impossible to you, but a lot of what we have today seemed impossible 50 years ago, so i donīt really get your point.
Whatīs plausible about The Joker scene? The man uses a rocket as a vehicle. If you canīt see how dumb and absurd that is, no matter how you twist it, thereīs not much i can tell ya.