With microcore, you have to explicitly load the X related stuff.
Try adding
Xlibs.tcz
Xprogs.tcz
Xvesa.tcz
wbar.tcz and
flwm.tcz
to onboot.lst. This should give you exactly what you would have with tinycore instead of microcore, so if you're not substituting something (Xorg instead of Xvesa or some other wm instead of flwm, for instance) you might just as well use tinycore.