Lolking and winrates often have little to do with whether a champion is good or not. Take Olaf for an example. His winrate was under 50%, said Lolking. But then he got used in tourney level play and destroyed things there. Then he got nerfed. Why could this be?
Obviously, Lolking doesn't factor in player skill, the champion's allies or the enemy team's performance. Nor does it take into account trolling, afking, or whether you were building the correct items for the game in question. So why use it for anything? Sure, if a champ has a very low winrate, there may be something wrong with them, but Lolking cannot tell you what that is.
As for Jayce, he's fine. He's got two different build paths (AD caster or bruiser), has excellent poke damage, good burst at all stages of the game, and also has one of the stronger laning phases in a solo lane due to his ranged harrass and ability to follow that up in melee, much like Nidalee can.
He might be relatively tricky to get the most out of, but having a winrate under 50% doesn't actually make him bad. If you're going to use winrates as a judgement of power, you also need to identify weaknesses in the champion to see how bad they are. If you look at Fiora (who has a terrible winrate), you can clearly identify a large number of serious problems she has (forced to build damage, but melee with no real durability. Weak laning phase, weak passive, and a need to snowball to be a threat). When you compare those to what Jayce has (which are mana - solved with a manamune, and being relatively weak when spells are on cooldown - like any AD caster), you see the difference.