You could use XSLT to output HTML - but -
think about what is in a Google sitemap.xml file
<?xml version="1.0" encoding="UTF-8"?>
OK, so I can change that to a nifty HTML page. What do I have that I can work with?
The "loc"ation certainly. But is it going to help visitors and would they even care when the page was last modified, how often it changes, or how important I think it is?
I could still add a helpful description and more meaningful link text, but no friendly URLs for bookmarking.
IMHO HTML site maps are a very good idea. But by the time you polish up the Google sitemap.xml you might as well have spent the time on another file no?