All champs have a high skill cap. It's just each champion requires a different skill set.
Sure, some champs can be easier to function decently with, but there's always a higher level of play that's obtainable with them. As an example, champs like Blitz or Ali have low item requirements to maintain usefulness, so they have fairly low skill required for last hitting. However, they require good knowledge of how to initiate and require good timing and enough experience to predict enemy plays. In that regard, you can notice the difference between a good Blitz or Ali from an average Blitz or Ali.
It's the same thing with other champs; it's either high item dependency, which requires good farming skills, or high "game sense", which requires knowledge of the game and of your opponents, or even both item dependency and game sense combined. However, each champ requires about the same amount of effort to get good at, it's just in what way that effort has to be applied.