You have to understand that making skins isn't just about the champion with the fewest number of skins. There are a lot of economic factors that influence what skins get made and who they get made for, things like how popular the champion is, how much work goes into making the skin and then how much work does it take to make a skin for that specific champion, along with standard economic factors such as marketing and pricing. You can't just say Zac only has one skin that's essentially a recolor so we are going to make three skins for him all in a row to give him some variety, it just isn't that simple. Plus, when the skins are made for certain events or holidays, the skins need to be able to match that occasion and fit the champion they are being given to. How much sense does it make to give Aatrox a skin based around the lunar revel? Or Kha'zix for that matter?
I agree that certain champions do need some attention as far as skins are concerned, but I feel like just numerical variance isn't enough to ignore making good skins for champions that they fit with, and other champions not having as many, doesn't warrant saying that those skins or the champions that received them are wrong.