No man, Amelie in battle is there just to look pretty. But every skill you learn with her, every item you have in inventory, every medal you achieve give some bonus to your army. Check in battle : right click on some of your troops and see their stats attack/defense/speed. . ., and later change items you have or learn some skill, and check again. You`ll see that all in hero screen have use and some bonus toward army you are leading.
Warrior Amelie have more attack bonus, and gain more rage to use in battles, mage Amelie have more intellect and double spell cast with her skills, paladin Amelie have more deffence and resurrecting her lost troops after battle. Every class of Amelie can cast spells and use rage for pet dragon, but warrior is strongest with big army and lots of rage, mage with high intellect spells, paladin for ... well i dont like paladin class much but i`m sure she have some use
If you pick warrior already you`ll have largest army, highest crit% and therefor most dmg from your troops.
Good luck