[Faceman]

yip,
Imoen has the thief avatar because she is a dual-class and so robes won't show. Leather armor however will (to some extent) but disables your spells.
The mask is the only helmet thieves and mages can wear and its use as a helmet is kind of cheesy (it was meant only as a quest item and to be useless afterwards)
Why the mask is showing on pure mages (which shouldn't wear helmets either) I can't say.