Practically everyone else does have a dress code. The Council, for example, all put on their formal dress robes when in formal session (this is usually to bring Eileen to account for her latest act of defiance), though they don't wear them for normal meetings. Naturally Brankaresh as Chief Wizard (and in his view Chief of everybody not royal) has the finest dress robes of all. Not that it impresses his nemesis, Eileen.
L'Evallier will don fresh formal wear for his audiences with Roxannadrell. Having said that L'Evallier never wears casual. Even his night attire has a formal look about it. He believes he owes it to his class and his species to always be "properly dressed". The downside of this is L'Evallier has no need for a "gentleman's personal gentleman" so would have no need to employ someone like Wodehouse's wonderful Jeeves. Very much L'Evallier's loss I think.
Roxannadrell herself considers her working uniform to be a sweeping gown with tiara and suitable accompanying jewellery (suitable depends on what event she is attending).
Eileen only wears formal gowns if dining with the Queen and wanting to either impress her cousin or mollify her a little by making more of an effort. Eileen is much more at home in casual but smart separates, especially when flying, and she has no time whatsoever for the traditional fairy costumes often depicted in our fairy tales. She rightly thinks these are not practical and would offer no protection against cross winds when in flight. Anyone flying on a regular basis would want to consider that.
Hanastrew wears smart casual while working but likes to put something a little smarter on when she finishes for the day. It acts like a signal to her brain she can switch off now.