I’ve popped the covers off a ton of J-series with high miles. That looks normal. What you want to look close at is the rollers on the rockers that contact the camshaft, and what both of those look like. They are the first to normally fail. From the few I can see, the rocker arm rollers look good. The reason the passenger side of the head is dirtier and darker than the other is because there is a valve cover breather on the far left that goes to the intake manifold that will suck the air and other crap that direction from the top of the head.