In order to make your Superstars more effective in battle you need to level them up and evolve them to the next star rank. 

Superstars unlocked through rewards or loot may will have a higher talent capacity than those that you evolve from lower ranks. 

Example: A 2-star Bronze Kane you unlock through loot or rewards will have a higher capacity talent score than a 2-Star Bronze you evolve from a 1-star Gold Kane

You can tell a Superstar was evolved to the next star rank if their second or third star appears only outlined instead of full.