Can be normal. Children have appetites that vary. As toddlers (12-24 months in particular) they will have days they eat like an adult and days they eat not at all. As long as baby feels good and acts normal just monitor appetite.Be sure he/she is drinking to stay hydrated. Sometimes appetite decreases when teething or if they are feeling badly.
Many reasons. Babies can go through up and down appetite periods, just like everyone. But, if it is a significant decline, if your baby isn't growing well, seems sick in any way, or just isn't acting right, you should see your pediatrician.