check if your laptop firewall does not block incoming X traffic and n770 is authorized to use the xserver (xhost +)
So now you did it the other way? laptop application on n770 screen? dropbear x forwarding has nothing to do with this when you ssh from laptop to n770. Or did you ssh from n770 to your laptop (via dbclient command)? Also I think xserver on n770 is not built (or started?) with TCP/IP support so you can't display anything remote on n770 screen. I'm not 100% sure about this, please correct me if I am wrong
Looks like I would need 'xauth' on n770 for this to work.