Are dog ears after mastectomy common? is it a serious surgery?

Uncommon: If by "dog ears" you mean an area of gathered skin near the corner of an incision, then I'd tell you they are uncommon. They can be an eyesore and occasionally be irritated by contact with garments. The operation to fix them is not a big one, but all operations are serious. My guess is that you've already been through a lot. If the dog ears are bothering you, talk to your surgeon about fixing them