Can I put neosporin on baby's belly button after cord falls off? To heal it?

YOU MAY. For 2-3 days, no more please. Seek advice if not healing beyond that.
Unnecessary. Multinational studies have shown no need for topical antibiotics on the umbilical stump. If anything an ointment can trap moisture in the area. Letting the area dry out completely and keeping it clean has the same effect & chance of any localized infection.