System.ini and Multi-Cart Board.ini are not being displayed in MAMEUI and other similar derivatives. I don't know why.
Also, you should replace [*Mature*] with [ROOT_FOLDER] in the Mature.ini file... because there's only one subcategory, which is Mature games... it doesn't make sense to have to click the Mature folder and then click on the Mature subcategory in order to view the games. Ya know what I mean?