My understanding of the dropbear documentation is that only the dropbear server (x11 client) can handle x11 forwarding. the dropbear client (on the computer with the X server wanting to be used) is not capable of x11 forwarding. When I first compiled openssh on TC, I made a few attempts to use x11 forwarding with openssh on my laptop and dropbear on my server. Those attempts were not successful, so I just put openssh on both ends and all is well. Personally I only use openssh for x11, although it is capable of quite a bit more.