My post from another thread:
Lux (in game at least) has always felt kind of like you're playing a teenage girl that happens to be crazy powerful. Spell thief and Sorceress skins play off this and fit into the theme well.
This is in contrast to someone like, say, Leona, who is this gritty, battle-hardened warrior ready to lead the charge into battle. She's a plain ol' badass army chick ready to kick ass.
Imagine you made a Leona skin in a schoolgirl outfit and she's acting like a bubbly tween. I'm pretty sure everyone who's played Leona would consider that a mistake of a skin. You'd be trying to smash Lux's personality on Leona. It doesn't work.
Steel legion (and to a lesser degree, her other military skins) seem to take the Leona theme and try to smash it on to Lux. She's walking around in this massive suit of armor. She looks like a freaking tank. When she fires bolts of light from her staff, she poses like she's holding a minigun.
It just doesn't fit.
Suprise Party Fiddlessticks definitely DOES NOT fit Fiddle's theme, but it's his most beloved skin.