In Breaking Bad Jesse and his junkie friends always wear knitted caps, even when it is really warm and nobody else wears anything like that. Also when Jesse returns from the rehab, he no longer wears a knitted cap, even though Badger and Pete still do.
Does it have any meaning? Is it a fashion thing, or does it indicate some medical condition caused by drug abuse?
It's just a fashion choice. Knitted caps are very popular among kids from urban areas in the US. It's very prevalent in hip hop culture and in association is considered cool by many young people, and Jesse and gang are obviously following that trend. You can see that trend in the way they talk as well, especially Jesse.
Not having Jesse wear the cap anymore is a visual queue that is very easy to pick up which signifies change. Just looking at Jesse you'll get the feeling that he's a changed man.
