Fertilization is unrelated to this type of dieback (and not needed or useful for this situation), but it's hard to diagnose precisely since the damage likely occurred some time ago, even if the canopy is only manifesting symptoms in recent years. Both drought stress / under-watering and poor drainage / over-watering can lead to canopy dieback in trees, as can planting too deeply or over-mulching. Maples, as an example, are prone to developing girdling roots when the root flare is buried. The tree pictured does look like it's either planted too deeply or over-mulched, but it's hard to tell since we can't see the trunk base very clearly at that distance. You can use the information in the pages linked above to look for the location of the root flare, to make sure it's sitting just at the soil surface.

Few issues bother Japanese Maples aside from growing conditions, but among those that do, Japanese Maple Scale can cause dieback if they are abundant. Wood-boring beetles can attack trees under stress from being too wet, and Botryosphaeria canker can attack trees under stress from having gotten too dry. None will be treatable, but dead branches can be pruned out as soon as they are noticed. The linked page describes (and a video link demonstrates) where to make the cut so the wound seals-over well.

Lichen is merely indicative of more light reaching the bark, since lichen photosynthesizes for energy. By itself, it doesn't necessarily indicate a problem, and the lichen isn't harming the tree.

For now, all you can do is to prune out dead wood and monitor the tree for watering needs as we enter summer and potentially dry stretches of weather. If the tree's roots are in decent health, eventually new growth can help to fill-in empty areas after the dead branches are cut out, though this process will take several years.

Miri