First of all I feel I should say that this is Ironstylus's design and therefore it's his own choice how heavily armored she gets to be. If you aren't a Riot designer then you are simply along for the ride. If you have an issue with a champion don't buy that champion, you are not a special snowflake, you are one of the herd, your opinion on the appearance of champions surprisingly is of no importance to the design team.
Secondly I invite you to read her full journal before judging her clothing, towards the end of the journal it says she was granted a place among Demacia's elite. Going from being a person of a lower class to a member of the city's elite would be something of huge importance to her and she would most certainly want to look the part, especially if she was granted the honor by the king himself. Her somewhat extravagant armor is a reward and a status symbol. For the people who say it is not fitting to her role as an assassin/huntress style champion take a closer look at it, it is mainly leather or fabric except at main joints and points of weakness, such as the knees, neck, shoulders and elbows, this would provide her with mobility as well as protection from slashing attacks or crushing attacks that humans are incredibly susceptible to.
Tl;Dr : This is Ironstylus's design not yours, you are not special and you do not deserve development fanservice. The armor is a status symbol for her new place in Demacian society and is very effective for a mobile, ranged based champion.