I use the Blucher NPC mod to change Imoen to a thief/mage multiclass? RPing wise that seems what she would be RPing wise, just wanted some other people's opinion on this.

I think she was dual classed in order to satisfy timeline consistencies to BG1.

She couldn't multiclass due to racial limitations... She's a human.

In RP terms, both the Dual and the Multiclass seem justified, though I personally would have held off the Dual a little longer, enabling Imoen to acquire the skills to help out the party AND still dabble in her favorite pastime of pickpocketing.

I would respect the race/class restrictions and keep her Dual-classed; both for the sake of abiding by the rules and also to avoid invading Jan's turf.