I can not help but disagree with your logic, I will not fight you if you'd rather Lily be a Rouge rather than a Theif, but I feel I should at least explain my logic now lol. 

I was under the impression that surface aspects of ones personality would not be quite as reflective of the class. I believed that the more deeper, core aspects of a characters personality would reflect in the class. For example Jade and Damara are both witches, but their surface personalities are quite different. Heck Damara sounds like she was NEVER like jade, before their game she was meek and soft spoken and later she is an aggressive, foul mouthed, mean spirited type. Jade is not either of those. Equius and John are both Heirs, but are very different in terms of surface disposition. 

I feel like the class more represent internal motivations, and this kind of comes from what I understand to be the difference between a thief and a rogue. Thieves steal for themselves (We never see thieves share what they have stolen) and rogues give what they steal to others (Roxy trying to summon something for someone else ). Not saying they both couldn't do the reverse, the game has proven itself to be very flexible in the abilities of the god tier, but it's probable not their natural inclination. 

Under this logic thieves will have a very 'me' focus, in a good person this will be shone as confidence in ones self and having a strong sense of who they are, in the worst this would make someone very selfish (vriska). Lily does not often share what she has crafted, she keeps most of her work to her self, showing stuff only to close friends if it is something she thinks they would be interested in. In this way she exhibits a very 'i do things for myself' core personality trait in an un dick-ish way. That is why I felt thief fits her.

As for the clothing/fassion thing....Yeaaaaaaaaaaah...I couldn't help it. I wanted to use Lily, and she is just a crafter who dables in design, so its a fun refference to the space players xD