Is it normal to have a stitch coming out of your nipple?

Thanks for your question.
The simple answer is, possibly.  It all really depends on the technique  and sutures used by your surgeon.  The majority of breast reductions are closed using a combination of absorbable sutures.  Both deeper and more superficial under the skin.  It is not unusual to have one of the stitches work their way up through an incision and start to poke out.  They will usually dissolve on their own but if it becomes a problem they can be removed without much difficulty.  
In some rare cases I have seen a non-absorbable suture used around the edge of the areola.  Usually this is seen in breast lifts, not necessarily a reduction.  Having this suture poke through the skin is not a normal event and can lead to issues with infection.  
Follow up with your surgeon and they will be able to tell you if there is any concern.
Best wishes.

It is not uncommon for patients to experience surfacing sutures post-op. This entirely depends on the technique and type of sutures used by your surgeon.  Regularly follow-up with your surgeon to prevent any complication.