I wanted to know if I did yoga, many poses stretch the neck area so can this affect the healing of the donor scar?

At a month, you can get back to your normal routine.  There are really no limitation as to what you can do activity-wise.
I do counsel patient to listen to their bodies though.  When undergoing an activity weeks or months after a transplant, they may feel tightness or pain when doing certain activities.  I encourage patients to lay off of those activities if you body is telling that it is still tight.  One may technically be fit for all activity, but I like to be extra cautious when it comes to this.

Flexing your neck shouldn't put undue tension on your closure, though it does take 6-12 months for a scar to mature and tension can cause scars to spread.  If you are concerned, it may be worth a visit to your surgeon to demonstrate the positions you use so he/she can assess how much stress is placed on your incision and see if there are any maneuvers to avoid.

After a surgical incision closed with sutures the following stages of healing take place. After first two weeks 25-30% of total strength of scar takes place, then next two weeks bring another 15-20%, then every week thereafter will give you decimentlay more strength. The scar will be completely resolved around six months. So during the healing process any upper body exercise will tighten neck muscles and pull against the incision scar causing widening scar tissue.