Actually, considering the two characters that are Mages in canon, I'm inclined to say that Mage is a Passive class rather than an active one. Mainly because our two mages (Sollux and Meulin) have very active personalities- Sollux in terms of his combat capability with his psionics, and Meulin with her more or less meddling/shipping all her friends (and pretty much everyone/thing else) ALL THE TIME- so giving them passive classes in the game would have been more of a challenge for them (as S---B is prone to doing) than giving them an active class.

Also in regards to the Witch/Mage active/passive pairing, I can see it both because those are two 'magic' related classes that are opposite each other in terms of terminology- 'Witch' being female and 'Mage' being more male/gender neutral in terms of assignment.

tl;dr- I feel Mage is a passive class due to canon personalities/occurrences.