The Xmir package needs to depend on x11-xkb-utils, particularly for the phone. When just installing xmir on the phone and then trying to start an X app, the following error occurs:
ARM architecture: Defaulting to software mode because glamor is not stable^M
Realize ROOT window 0xb8f4a9e0 "": 768x1280 +0+0 parent=(nil)^M
depth=24 redir=0 type=0 class=1 visibility=3 viewable=1^M
override=0 _NET_WM_WINDOW_TYPE=0 WM_TRANSIENT_FOR=(nil)^M
sh: 1: /usr/bin/xkbcomp: not found^M
sh: 1: /usr/bin/xkbcomp: not found^M
XKB: Failed to compile keymap^M
Keyboard initialization failed. This could be a missing or incorrect setup of xkeyboard-config.^M
(EE) ^M
Fatal server error:^M
(EE) Failed to activate core devices.(EE) ^M
XMir has closed unexpectedly^M
After installing x11-xkb-utils, then the error no longer occurs and the X app will start.
You don't have xserver-common installed on the image? That would be the correct fix.