To show only the subcategories when you select the three bars - mobile menu that ships with Zen Cart 1.55f

includes/templates/responsive_classic/templates/tpl_modules_mobile_menu.php

Replace line 33
<li><span><?php echo BOX_HEADING_CATEGORIES; ?></span>

With this:
<!-- Remove first tier of mobile nav categories box <li><span><?php echo BOX_HEADING_CATEGORIES; ?></span> -->

Now the top category will be hidden and the subcategory will show.