I tried to summon a giant with a player head, using this command:

/summon minecraft:giant ~ ~ ~ {ArmorItems:[{},{},{},{id:"minecraft:player_head",Count:1b}]}

And the player head is not visible, although it should work like how it works with other mobs, like zombies and baby zombies, husks, skeletons, etc.

The weird things, is that it does work with helmets, using this command:

/summon minecraft:giant ~ ~ ~ {ArmorItems:[{},{},{},{id:"minecraft:iron_helmet",Count:1b}]}

And this works perfectly as expected.

(see picture: on the left, a giant with a player head, not working; and on the right, a giant with an iron helmet)