Any driver needs a good car to win races and ultimately the title, that much is true. Put the likes of Hamilton and Vettel in the current Williams and they obviously wouldn't win races in it. Sometimes drivers outperform their cars, that is also true, but having the best car doesn't automatically win you a title, if it did the likes of Coulthard, Irvine, Massa, Webber, Barrichello, and Bottas, to name but a few, should all have been champions by now and none of them have been.
To be a champion you need that extra something which only some drivers have. Any great sportsman will tell you that a lot of the reason you win is to do with mental attitude and that is true. Some days you can find that supreme focus and you're so on it, nothing can stop you. Other days you can't do what should come easily to you and you can't put your finger on why. If your head is not in the right place you can more or less forget it.
Hamilton has had meltdowns in the past, he can lose it big time if he's not right mentally, it's happened time and time again. Just watch him if he fluffs quali or in the race and the pressure on him ramps up. Sometimes he can pull a great lap out when it really counts, but all too often he'll make an even worse mistake. That is not a criticism of him, he's a human and that's what happens. Right now he's in the fortunate position of knowing he has arguably the best car on the grid and he has a great team behind him, it's easy to be focused and doing well.
Right now, for whatever reason Seb's head isn't in the right place. Maybe it's too much pressure from Ferrari, or from the fact he has a new strong team mate, or maybe he's putting too much pressure on himself, or a combination of all three. Who knows? I reckon until he sorts himself out he's going to keep on making these stupid errors because he's trying too hard not to, and that is causing all the more if that makes sense. It's a vicious circle.