In Moin 1.7 and newer automatically only the category key is displayed. There is no need to apply the following changes.
In Moin 1.6 you can configure that categories should be displayed without the prefix Category. To do this, change the regular expression for categories in your wikiconfig.py from u'^Category[A-Z]' to:
1 page_category_regex = u'^Category([A-Z].*)'
As the Explorer theme displays only the first matching group of the regular expression, the prefix Category is omitted. If you have modified the regular expression for categories, you will have to change it accordingly.