The one-sidedness of the game has less to do with skill level and more to do with the game's importance on stats and snowballing as core mechanics.
The skill cap imposed on everyone in the game (that being you can only position so well and use the few abilities you have so well up to a cap) typically means most fights come down to stat advantage. Just because someone is fed doesn't mean they are skilled. Thats an important key note to remember. It could mean any number of things, like a enemy's single misstep, a successful gank, a lucky break, countered picked his opponent, or simply having more stats from the get go (like picking a champion with higher base DPS) thus out trading your opponent because you blow for blow deal more damage than they do.
That said, the player all in-ing you could be much less skilled than you are, and you would tear him apart in a even fight, but he will absolutely rofl stomp you because you simply don't have as much stats as he does. Hell, he and his opponent could have been equally less skilled than you, but because he got a step up somehow, he won and now he's bearing down on your whole team.
EDIT: Before someone gets the wrong idea, YES skill means a fucking LOT, but skill matters the most when you are even, I.e in the beginning of the game when everyone is most equal in stats. Once someone gets ahead, stats become secondary to skill unless you have so much skill to compensate, but the skill cap in every champ limits you to do even that. The most you can do to demonstrate skill while behind is not feed and farm stats as efficiently as possible. Straight up outplaying and killing a more fed opponent on your own is almost impossible in this game.
