I personally don't take issue with the armour in the game. There are female champions that have varying degrees of practical armour just as the males do. I think once it was said that in order for a character to exude the fact that they were female their "assets" had to be more pronounced. While there is some issues with that, I can see the general point of it.
I think it's pointless to argue armour practicality in a game based around something as non-practical as the ability to preform magic.
There are some amazing examples of females who have amazing armour sets that aren't really excessively impractical or sexy, such as Diana and Quinn and as for the mages, it's standard in any fantasy roleplay or mmo or anything like that for mages to wear less armour than melee or physical damage based classes because they are meant to hang back and it hampers their abilities.
The only thing I wish Riot would provide their female characters with is a variety of body types and ages. While we have Annie, we don't really have an old hag of a character, etc. Rek'sai was an amazing step in the right direction and she is a loved champion (though she sometimes still gets referred to as a he). I think there's lots more that can be done.
I think armour is the least of concerns anyone should have about the game. While is has merit somewhat, if you want to really make a difference in armour practicality you have to also include the males in this game, as a lot of their armour makes absolutely no sense either. I for one, prefer the interesting differences that can occur from having both practical and impractical armour in the game....if we didn't everyone would look the same because there's only very minute differences that can occur in armours and have them still make sense in a real situation (which League of Legends is not).