This looks to me like a combination of too high heat and crappy tip coating. Unfortunately this is common with unregulated soldering irons. They usually get much hotter than a regulated iron because they always put out the same power, and that power has to be enough to solder against a ground plane or something else that acts like a heat sink. A regulated iron would just crank up the power when needed, but a unregulated iron is stuck with a fixed power, so gets too hot, sometimes way too hot, when just sitting in the holder. I've seen a unregulated 25W soldering iron get hot enough to visibly glow in a dark room.
The excessive heat can not only damage whatever you're soldering, but it speeds up the oxidation reaction that eventually ruins tips. Solder oxidizes fairly quickly, even at normal soldering temperatures. You may have tinned the tip regularly, but I'm sure that didn't last long. Then the tip coating starts to oxidize. Once that's worn off, you're left with copper. Copper is actually eaten away by soldering itself. It migrates into the tin/lead mix, which is one reason solder works so well on copper.
People that buy unregulated irons are shopping on immediate price, so manufacturers make them as cheap as possible. That probably includes the cheapest possible tip coating that looks good out of the box.
So in short, it looks like you got what you paid for.