She needs another chance to shine.

I was thinking, since Joshua has a strong affinity for Darkness and can therefore use Dark spells, why can't Amaya use light-enhanced spells given that she's a princess of heart and therefore has a very strong connection to light.

These Light spells are weaker then their normal version against normal enemies but against Heartless and those with a strong connection with Darkness (Joshua included) take damage equal to a buffed normal spells. Unlike Dark spells, which focus on offence and destroying everything around them, Light spells are defensive and grant buffs to the user and the party when they hit an enemy.