I have spent a lot of time reading and trying to understand the various confusing and sometimes contradictory comments made about whether or not bbPress is a responsive design or not.
As best I can figure out from that material, bbPress is not a responsive design. Rather it relies upon website themes being responsive. However, even if a theme is responsive, that does not mean that a bbPress forum will be responsive.
People have posted information about lots of problems they have had trying to use a responsive design forum with bbPress. Some people have also posted various technical workarounds to at least some of those problems (which I can’t understand).
So, the bottom line for an ordinary person like me, is that there is no point in using bbPress as a responsive design forum.