No vaccine causes loss of appetite, few vaccines (DPT)might cause fever n discomfort but no such effect seen with MMR.
Next Steps
Try to change with some alternate feeds, n If actully the baby is not eating well , then plz consult ur pediatrition in person. Don't connect it with vaccination.
Health Tips
Baby might have blocked nose or cold causing low appetite,
which can b taken care if u visit ur doctor.