Given we are seriously mentoining Morde, Kayle, Singed and Akali I don't even know what we are talking about? Moody mentoined Mages as If only they dealt magic damage to have chance to shittalk them. Somehow when it comes to physical damage we have Tanks, Bruisers, Skirmishers, Assasins but with magic damage it's all Mage? That sounds biased. And only half of the problem is Mage itemiation.
If we want to go with dominantly magic damage, as would OP's post suggest (since he talks MR items which counter magic damage not AP):
leagueofgraphs | #10 | Plat+ | Toplane | Going by common sense of what is majority of their damage from by "What would I build to def against them; Armor or MR?"
Pickrate:
8 physical damage, 2 magic damage (Morde, Tahm Kench)
Winrate:
6 physical damage, 3 magic damage (Kayle, Neeko, Maokai)
then there's Voli who is the only one I cannot categorize, so it's either 7:3 or 6:4
Banrate:
5 physical damage, 5 magic damage (Morde, Tahm Kench, Vlad, Sylas, Akali)
Physical damage is simply picked more often top while there are picks from magic damage people are frustrated with. From these only two are really Mage.
Going by pickrate (real dominance), only Supp (8) and Mid (6) are dominated by magic damage and even then Supp is only dominated by it because Rissoto games has allergy for physical damage enchanter (note that it's possible to scale with AD and still do physical damage).
Even If you go All Roles, it's literally 15:5 ratio in favour of physical damage.