Noelle's lust attack is special; it always does the same amount of damage, no matter what. She has no special skill - she's a spoiled, rich girl who's never worked hard for anything, or done anything but shop and flirt - so this is literally all she has going for her ;P
Also, I feel like I should point out that you don't have to seduce the Boss Girls. You can still recruit them via the dungeon if you beat them through physical attacks.