I'm assuming you're talking about things like these? 1, 2, 3 In actuality, they're not that far off from Spanish drawings of headgear. (Also see pages 104,106,110,112...) These are each Inca kings (Sapa Inca), so the specific headgear is likely more fancy than the average footsoldier would use, though the construction is similar. Other images do show other military officials (here with spear on the left and shell trumpet on the right) wearing the same as the captured rival Inca. Unfortunately, the artistic tradition of the Inca homeland in the south-central Andes did not often feature human figures, so we can't exactly look to that for more evidence. However, the earlier Moche on the northern coast painted detailed scenes of human life on their vessels. From images like this (pg. 167, warrior #15) we can see that it is not an unprecedented form.

The first drwaings all come from a fellow named Felipe Guaman Poma de Ayala. He was born in 1535 into an elite native family, which had maintained its prestige through smart political play as they transitioned from submission under the Inca to the Spanish. Poma'd greatest work, from which these illustrations come, was the Nueva Cronica and Buen Gobierno, both a chronicle of history from Adam and Eve on, and an indictment of Spanish cruelties to the local peoples. The work is very helpful in reconstructing the succession of Inca rulers and provides of a number of great illustrations of life at the time. While some might seem a little cartoony, some are very helpful for questions like this.

There isn't really anything to special about the helmets. They were wooden or copper, and the feathers were decoration.