What are they like? How are they treated? How are they viewed?
Every region has a mage, and in the lore, every region has at least some mention of how mages are viewed, and their roles in society, except for Zaun and Piltover. Yes, I'm aware for gameplay purposes that Viktor is classified as a mage, but in the lore, he's just a scientist, albeit, an incredible one. On the map, when you clcik on Piltover and Zaun, all they say about magic is commodify and exploit respectively, which doesn't really tell us much of anything.
Where in the Freljord Mages are shamans and priest, and in Noxus they're simply weapons of war, my belief is that in Piltover and Zaun Mages are viewed as artificers and artists. Hextech, which is a huge source of income for the dual city states, is in itself magic, as it uses the magic found in crystals to create their devices, and with a Mage being somebody who quite literally manipulates magic, it would seem quite beneficial to have one around. If anything, Mages in Piltover probably work the same way an artist would, by seeking a patron and offering up their skills and arcane abilities in return for wealth. Perhaps they work more like batterys, helping in the creation of artifical hextech crystals, and as we know from Camille's lore, articial hextech crystals are something that can be made, and is even what they made Clan Ferros so rich. We also know that Piltover does have a class system, the city itself ruled by the Bluewind Court, an aristocratic oligarchy. Where marriage or having great skill with hextech can allevate you higher in this system, it would not be to farfetched that being mage could also do the same, with some of the common folk maybe even hoping their child be born a mage to help them reach higher.
If this were to be the case, then in the depths of Zaun, no doubt many would pray feverishly for their child to be born a mage, so they could have a greater chance of being able to escape living in the slums of Zaun. However, even though it doesn't give much to work with, it does say that magic is "exploited" in Zaun. This could mean that, although being born a mage might be a more fortunate thing, it puts a target on you, with mages from Zaun who couldn't make it up to Piltover being taken up by the Chembarons that control Zaun, and forced to work for them and help them create their own hextech.
Of course, this all just my personal headcanon and purely speculation and assumption. Something I would like to see League take their own personal twist on is the idea of magical clans. Maybe in Piltover, Mages marry other Mages in order to continue on their their magic, and it would be quite unique to see the normal aristocracy with a magical aristocratic society right below it, as in most fantasy, you either have mages ruling or nonmages ruling, and never shall the two meet.