All those Windows Media Player skins were awful because they used so much screen real estate on dead space. Whereas the plethora of Winamp skins kept the economy of screen real estate while still providing unique and imaginative visuals.
The whole skeuomorphic trend starting in the mid-90s was similarly awful for the same reason. First, it was often hard to tell what was a control and what was just decoration. Second, it often took trial and error to figure out what was what. And, as I mentioned above, these designs almost inevitably wasted huge chunks of screen space on decoration that provided no functionality.
Of course, we have the opposite problem now. All windows look the same. Title bars are mostly gone. And since companies like Microsoft replaced all their HCI experts with art-school dropouts who think the "flat" look with low contrast is cool, not only can you not tell what app you're looking at. Half the time you can't even tell where one window stops and another starts.
The only good UI thing that's come out of the last decade or two is a near universal support for "dark mode". Otherwise, I would greatly prefer the Windows 2000 "classic" look, or something similar.