The forum is all right on mobile (I use it roughly 75% mobile), only these little things that disturb:
1) the text input box does not adapt to resolution so I have to scroll sideways writing / editing or insert hard line breaks all the time
2) The "compose message" and "open thread" windows are so cluttered that I have one line as input and this line is scrollable left-right
3) Some linked images do not resize properly and need sideways scrolling too, but this is minor.
Conclusion: To make the forum very nicely accessible on mobile devices also those with "only 1200x800 qpix" resolution the only thing that has to be done is:
-- Possibility to get rid of three lines of clutter (B) (Mona Lisa) (Smileys) in my settings
-- Textbox adapts to screen resolution, so no sideways scrolling necessary.