What is the origin of the term 'Mana Dork'?
The oldest reference I can find for this kind of usage of the term "dork" is an event coverage article on the Wizards website apparently dated January 1, 2000. It contains this description:


  "dorks" - creatures that are "ok" but not "savage"


This may be establishing the use of this word in this way, or it may just be explaining the usage.

The oldest reference I can find for "mana dork" is this MTGSalvation Forum post dated August 29, 2005. So, it seems likely that "dork" came first, and a "mana dork" is just a "dork" that produces mana.