I wonder how much of this is nostalgia. Winamp was awesome, but given that skinning was part of its popularity, form was definitely a big deal.

Winamp skins were surprisingly restrictive, though; you couldn't really skin Winamp in a way that impeded its functionality / created mystery-meat navigation. Every Winamp skin was fundamentally just a set of textures applied to the same standard controls layout: https://skins.webamp.org/

Windows Media Player, meanwhile... https://www.theskinsfactory.com/wmpdesign

That was true for "classic" skins, but starting with Winamp 3, Winamp supported free form "modern" skins that were defined in XML and had a custom scripting language so that the author could define their own behavior.

True; though I'm pretty sure the WinAmp anyone here actually has any level of nostalgia for, is WinAmp 2. WinAmp 3 was so lacklustre that they brought back a lot of WinAmp 2 stuff and called the result WinAmp 5. (And then everyone continued to use the Classic skins, which WinAmp 5 supported.)

If the form impeded function, it was easy to change. Some skins even made sure to cover all of Winamp's sections properly. The Invader Zim skin appealed to my 15-year-old tastes and still offered visible, functional controls with a readable playlist. Some skins looked cool, but if they removed options, they weren't used other than to show off to friends on occasion

The form could be changed, but the substance remained the same: an incredibly well-design app that just worked.