There's no reason Dr. Manhattan could not have cured Janey's cancer. For all intents and purposes, he has the powers of Molecule Man, a villain so powerful that only his psychiatrist can beat him.
Dr. Manhattan is nigh-omniscient. He can (generally) perceive every instant of time (though it may be limited to things he has/will/is perceive/ed/ing). He literally has known since he first gained his powers everything that was going to happen. He knew the second he reconstituted his body that he would end up, years later, standing on Mars building beautiful, doomed things. He knew that he would forget to give Janey air. He knew that he would forget about the everyday miracles, and that she would remind him.
In a way, Dr. Manhattan is not a character in Watchmen at all. Until the point at which his observation powers no longer work (which, in the comic, was due to tachyons that he postulated (or saw himself postulating) could have been caused by nukes detonating) he is completely unable to take any action excepting those he has already taken. He is, in essence, an actor in the play of his life. He has a script and it cannot be deviated from.
As he cannot truly choose (though he appears, to people with a strictly linear view of time, to make choices all the time) he cannot truly 'do' anything. He, from his viewpoint, has always freaked out in the studio. He has always been standing in a room of 'super heroes' staring at a woman he has always been dating. He has always been catching flack from his then-girlfriend for staring at the young woman he has always been leaving her for.
So, dependent upon your point of view, Dr. Manhattan can't cure anyone's cancer because he doesn't cure anyone's cancer. If he doesn't cure anyone's cancer, he cannot cure anyone's cancer. His powers, however, mean that he could have cured anyone's cancer, at any time.
It's almost like he's in a quantum state - he both can and cannot cure cancer. The only problem is that with Dr. Manhattan, all quantum states have been viewed. They have been collapsed, and there is only one possibility. Schrodinger's cat is not both alive and dead, he was alive or dead before he was even put into the box, but no one knew except Dr. Manhattan.
Once he regains his free will (due to being incapable of perceiving his future state) Dr. Manhattan can act again. But at that point, he doesn't cure cancer. Instead, he decides to become God.
TL;DR: He can't, unless he does, until he can, but doesn't.