Is target disk mode a clean way to install (Mountain) Lion on a blank mac?
You should be fine installing that way. The OS X installer is pretty flexible, and is designed to be able to install on external drives (which is essentially what a Mac in Target Disk Mode is). With the exception of the system disks included with Macs prior to Lion, which were machine-specific, OS X installs work on any Mac the OS supports. You can even boot a Mac from a different system's drive without issue, as long as it's not a version prior to that Mac's release.