I've never liked any Show-Desktop icon that I've ever seen in any operative system. For years I've used some Windows 7 icon that I colored orange, but I've never liked that one much either.
Finally I decided to make my own. First I customized some green wallpaper so that the Ubuntu Mate logo became somewhat bigger, then I over-sized my panel to 140 something. After that I just took a screen shot of a desktop with a clearly visible logo and a more or less visible panel, scaled it down and replaced the old one. It doesn't really look like a Show-Desktop icon like we expect it to look, but I know what it is and I like it:
<imgNow I've started smurfing with a slightly different one, more traditional. There's some perspective and there's also a little green pencil: