The trouble is, by default bbPress has a lot of bells and whistles (features) that will quickly clutter a small screen.
So it’s a lot of work to clean up the clutter and remove unnecessary design elements, borders, etc. Convert “px” to “em” and percentages. Use something like Flowtype to resize your text.
Here’s an example of what you can do: http://linkedexpertsalliance.com/forums/
I spent about an hour just cleaning up the boxes and borders bbPress draws around the text editor, which makes all of your editor buttons too small to press on a phone. You want that text editor to use up the full width of your device. I tried to make those buttons responsive but the problem is you need those buttons to be big enough to tap, like the “bold” button is tiny. In the end, decided to go with a fixed viewport. It’s just lots of little tradeoffs here and there that makes it frustrating.