Can ceramic coating fix faded or failing clear coat?

No. A coating can make lightly weathered paint look glossier for a while, but it cannot rebuild clear coat that is oxidized, cracking or peeling. Coating over failing clear coat also gives poor bonding and short durability, because the surface underneath is breaking down. Lightly faded clear coat can often be polished first and then coated. Clear coat that is cloudy, flaking or peeling needs to be repainted before any coating is applied.