Look in
classes/XLite/Module/XC/ProductFilter/View/TopCategories
See this line:
class TopCategories extends \XLite\View\TopCategories implements \XLite\Base\IDecorator
Since your class also decorates this class, I believe you need to override this by redecorating:
XLite\Module\XC\ProductFilter\View\TopCategories
Then you add a dependency in your module and/or to your class. That is where I need Tony to help explain better....
I am also interested in this subject, getting my module to override another one. It is happening to you because XC/ProductFilters come alphabetically after XC/CustomSkin. Don't worry it is certainly possible, I have done it but I cannot remember how or find my code at the moment. Tony? Alexander? Anyone?