No champion should require a large amount of skill just to play. Remember that a player shouldn't be punished for trying to play something that's above their skillcap. There needs to be things about every champ that are simple and straightforwards, but give the sort of room necessary for making higher level plays.
For instance, no part of Rumble's kit is inherently difficult to use. Press Q, you get fire. Press E, you shoot lightning. Press W and you go fast. Clicking and dragging for your ult is probably the most mechanically intensive part of Rumble's kit. But there is a CLEAR difference between noobs using Rumble, competent players using Rumble, and people who have actually mastered Rumble.
A noob will more than likely suck with him. His last hitting is awful early game, and he can be a bit janky to use overall. But the concepts are simple, so against other noobs you probably won't completely feed.
A competent player will practice Rumble until they've got a feel for his kit. They'll learn how to better manage his Heat resource, and when to take what skills. They'll be able to last hit pretty reliably.
A master is someone who's going to have a rune page JUST for Rumble. The type of player who can get 5 out of 6 minions, while his flamethrower is active, while also harassing the enemy out of lane. He'll be able to snipe a fleeing opponent over a wall with his ult, and know just went to charge into a teamfight while overheating himself to turn the tide of the skirmish.
High skillcap doesn't mean that you need a large amount of skill to play a champion. High skillcap means that the more skill you have with them, the more you will be able to do above and beyond the average player.
Take me for instance. I can play Lee Sin if necessary. No part of his kit is difficult, the hardest thing he ever really has to do is land his Q and make sure he's ulting someone in the right direction. But just because I know his combos and play patterns does not mean that I'm going to do nearly as well as a Lee Sin main, who could probably ward jump circles around me. And therein lies the difference. Not that Lee Sin takes skill to play, but that Lee Sin takes skill to make the sort of overwhelmingly precise and stupidly well timed plays that he's all too well known for making.