As you have found, yes it should work fine, providing that none of the extra apps and services installed by the desktop distribution are locking the ports used by zimbra. To put it another way, a Ubuntu desktop installation that does not have sql and postfix installed should be fine.
The reason most use server is that the additional load created by the GUI and extra services can be better used powering Zimbra itself, but if you are more confortable with the GUI and for testing rather than production, I see no issue here.
Come one come all, as they say
