Should I consult a doctor about my baby’s mouth sores?

Yes. Yes, especially if they're painful or interfering with feeding.
Yes. Mouth sores in babies are usually viral infections. Babies under 3 months of age should see the doctor to find out if treatment is needed to prevent spreading of the viral infection to other parts of the body (can be a serious problem in very young babies). Older babies with mouth sores usually recover on their own, but should be in contact with the doctor to be sure symptoms do not worsen.
Sores. If the baby is not eating well or having high fevers call you doctor to tell you how to proceed.